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Muzzle Blasts Online
Fine Tuning A Slug Gun
|Phil Orem prepares to fire a shot on the slug line.|
Before getting into specifics I'll emphasize some general principles that we should follow: Since each rifle is apt to be enough different from others, making it one of a kind, the fine tuning process takes careful, logical and skillful patience. Never change anything that can't be changed back to its original condition without consulting respected experts. There are several "Systems" for slug rifle accuracy, and each requires differences in setup and tuning. For our purposes this tune-up should be strictly confined to changes in factors such as bullet length, diameter, and alloy; patch thickness, width, length, and lubrication; powder make, granulation, and charge; and caps and loading routine. Leave the barrel alone and consult with a proven expert before making any changes to molds, swages, and loading tools. By the way, I consider an expert to be a slug gun maker who wins with his own rifles. This means we're down to about ten people in the United States whom I'd trust on critical matters. Too many fine original and reproduction slug rifles have been damaged by ignorant or overconfident shooters. Remember, in reality we are merely caretakers of a fine rifle for the next shooter. Isn't that why we got ours in good shape?
My first slug rifle, made by Bob Morris in 1965, was thoroughly tested and used successfully by him in competition prior to being passed to me. With his advice and Eddie Ballotts' coaching I was able to do well with it from the very beginning by just picking up where Bob left off. It wasn't that way with my second slug rifle, a .40 caliber, unchoked gun also made by Bob Morris.
When I visited Bob's shop to pick up the new rifle, he supplied all the correct tools and cautioned that since he hadn't shot it I'd be starting from scratch. This was unusual, as Bob won't part with a rifle he hasn't tested and used in matches, but then he doesn't build them for sale. I wasn't about to decline his offer of the rifle and preferred to rely on his assurance that all necessary assistance would be provided to make it shoot as well as his other record-setting rifles.
Here was an opportunity to collaborate in the testing of a new slug rifle. A thorough cleaning revealed a slight amount of surface corrosion in the same location in each of the gun's interchangeable slug and ball barrels. Bob recalled that years earlier a cap had been snapped on both oiled barrels to test the action and sealed ignitions. To correct this, he lightly lapped both barrels and double checked that the bore specifications were proper. Next the breechplug, ignition system, and tools were inspected. The rifle's bulletmaking tools were examined and tested. This rifle uses a one-piece bullet that is cast in a non-adjustable, split-block, base-pour mold, and then hammer swaged to finished shape in a knockout pin swage. Trial casting of lead: tin alloy bullets showed no casting problems. The mold produced good slugs, due in part to the 3/8" thick sprue plate that helped hold heat as the critical base edge filled out. I still didn't know yet whether the bullet fit the bore, or if its length was proper for the uniform rifling twist of one turn in 18.25 inches.
Once-piece slugs are more difficult to adjust to the bore than two-piece composite slugs. Composites, with a hard nose and soft base, aren't as likely to have their profile, length, and upset changed during firing, as are single pieces. The alloy in a one- piece slug must be soft enough to instantly seal the bore and not gas cut, and hard enough to retain the proper nose profile and upset length. Too soft an alloy will allow the nose to upset unevenly, contributing to unstable, inaccurate flight.
The rifle's bore, .4042" groove diameter and .3970" land diameter has grooves approximately .0035" deep. Experience indicated a 100-percent cotton rag bond paper patch .004" thick, coated with Dupont Series 851-221 Gray Teflon would work well in its unchoked, uniform-twist bore; based on the rule of thumb that the bullet's base diameter should be land diameter plus up to one patch thickness in total diameter. But this combination of .004"- thick patch and .394" diameter bullet barely met resistance when first pushed through the false muzzle together. Examination of the slug showed that there was very little rifling print on its rearmost portion. Also, the naked slug would fall through the false muzzle without the slight resistance which would indicate a land-diameter bullet. At .394" base diameter, the bullet was .003" under land diameter and certainly undersized. Before opening up the swage a bit to get a tighter fitting bullet, Bob and later Ernie Swain suggested firing to check for both length of upset and the correct bullet metal alloy to get good profile retention. Ernie recommended an alloy of 60 parts pure lead to one part pure tin, and groups of slugs were made of pure lead and lead/tin alloys of 60:1, 50:1, 40:1, and 30:1. A starting powder charge of 60 grains of G.O.I. 2Fg black powder was used based on the .40-70 Sharps, straight (case)single shot rifle load and an old rule mentioned in Ned Roberts' book The Muzzle Loading Caplock Rifle.
It suggests two grains per caliber for bores of .40 caliber and above, hence 80 grains; or one-and-one-half grains per caliber for bores below .40 caliber, hence 60 grains. Later I'd find out that 65 grains was ideal. All alloy slugs were allowed to sit three days before use to prevent age hardening from confusing the study of recovered, fired slugs.
The first firing was into a ten-foot-long, fifteen-inch-square plywood box filled with dry sawdust lightly oiled with reclaimed 30-weight motor oil. Remarkably undamaged slugs were recovered one at a time to avoid confusion during comparison. This initial testing was enlightening, and examination of recovered slugs and study of upset, length, and evenness of rifling impressions, base deformation, and profile retention showed important clues about what was happening to the bullets during loading and firing. The pure lead slugs sealed very well, but looked more like semi-wadcutter pistol bullets than the three-inch radius ogive slugs they were before firing. In fact, these pure lead slugs upset so far forward, and noses distorted so far rearward, that bullet metal contacted the rifling beyond the end of the paper patches, causing mild leading even though .003" undersized. These solid, flat-based .40 caliber slugs stopped in four to five feet of the oiled sawdust. Hollow-base Minie type bullets will penetrate much farther because their form stabilization reduces end-over-end tumbling in the sawdust, which rapidly slows the solid-based ones. Recovered harder alloy slugs had gas cutting in the rifling grooves, the usual place for it on a slug, but bullet profile retention was improved. With the 30:1 alloy, the fired bullets' profiles were perfect, rifling impressions extended approximately 45 percent up the upset slug, and pitting of the base from powder particles was roughly one half that on the pure lead test bullets. But these 30:1 alloy bullets were usually badly gas cut in as many as three places, as if they had been deeply gouged along the rifling with the point of a screwdriver. Sometimes the gas cutting ran full length in one place and 1/8" or so in another. I decided to stick with the 30:1 alloy because of its good profile retention. Using too hard an alloy is apt to create excessive stress on the tip of the bullet starter during loading, and use of too soft an alloy may upset well enough to compensate for an undersized bullet but produce the uneven nose profile mentioned earlier. Use of a saw dust box to recover fired slugs for study is essential; it can't be bypassed!
Since the slug's initial diameter was .394" and the rifle's land diameter was .3970", Bob's solution to the gas cutting was to slowly open up the rearmost twenty percent of the BULLET area of the swage to .3975". In doing this he checked progress often and ran patched slugs through the false muzzle until he got them to show uniform, distinct rifling imprint on twenty percent of the slug's overall length (on the slug itself) after the patch had been removed. This slug's diameter was now .3975", as he wanted. Fitting a bullet to a choked bore is more involved and may require a bit larger and softer bullet than for an unchoked bore. In increasing the base diameter of this .40 caliber bullet we had to be careful to avoid making it too large. Some bullets starters are so powerful that they can push oversized bullets into the bore, and the squeezing raises ripples on the bullet's base. Further test firing of thirty shots into oiled sawdust quickly confirmed that the gas cutting problem was eliminated by enlarging the bullet's base. Upset was for 45 percent of the slug's overall length and nose profile was maintained.
In a total of sixty shots over two afternoons and Bob's three hours of shop work, we knew conclusively that the bullet fit and upset properly, thereby eliminating a frequent cause of slug rifle flyer shots. Some slug rifle shooters struggle for years with well made files without knowing the accuracy gremlin is mostly bullet fit, even though weather, powder charge, scope adjustment, cleaning, and loading, bench design and technique or the shooter himself certainly have their part in the game.
Next came chronograph and target testing to work out powder charge, cleaning and loading routine, and bullet stability. Using an Oehler Model 32/61 Skyscreen chronograph, it was an easy matter to measure velocities and learn the effect of differences in shooter-controlled variables on muzzle velocity. While firing over the chronograph I also targeted the loads at 100 and 200 yards and examined bullet holes for signs of instability that usually show up as yawed, off-center holes. Groups were good at both ranges, confirming that no major mechanical problems remained. Later, groups fired at the 300-yard range at Friendship showed no yawing with the 3.2 caliber-long bullet.
Now that I had a well fitting bullet, more tests were run to learn the best powder charge and the causes of vertical stringing of shots at longer ranges. Slug gun shooters call vertical stringing "jump or drop shots" and they have been the cause of more than a few lost matches. Altogether, several hundred shots were fired in these time-consuming tests, and suspected influential variables were carefully isolated and changed one at a time to be as sure as possible of conclusions.
The first tests established an accurate load. These showed that as the powder charge was increased from 50 grains to 90 grains, the velocities rose at a decreasing rate, about 50 f.p.s. per five grains' increase until a point at 85 grains where shot-to-shot velocity uniformity was poor. Once the range of uniform velocity powder charges was known, a charge that fired the slug at a muzzle velocity of approximately 1300 f.p.s. was tested for accuracy and bullet stability. Several ten-shot strings showed that 65 grains of sifted G.O.I. FFg powder, giving 1275 f.p.s. muzzle velocity with gentle but firm pressure of the bullet on the powder, gave 10-ring accuracy at 200 yards. Since a safe 300- yard range was not nearby, the tests then switched to determine the causes of and ways to control muzzle velocity variations, and hopefully, drop shots.
A few hours spent with the ballistics tables in Hatcher's Notebook (available from the National Rifle Association) and an electronic calculator revealed that small differences in shot-to-shot muzzle velocities with our low-velocity bullets could theoretically move a hit out of the 10-ring at 300 yards. In the case of the .40 caliber rifle, these figures indicated that a plus or minus 20 f.p.s. spread in muzzle velocity could account for up to a three-inch change in point of impact at 300 yards. A series of tests was run involving differences in powder charge and lot, bullet weights, caps, patch width and thickness, cleaning routine, and loading pressure. Space does not allow an elaborate review of the results, but the conclusions are quite clear: The most influential factor for both my unchoked guns was bullet seating pressure and its uniformity. If the bullet was seated on the powder hard one shot, soft the next, and then with moderate force, a 50 f.p.s. to 60 f.p.s. difference in muzzle velocity was repeatedly produced. Seating a string of shots uniformly hard, soft, or somewhere in between minimized the variation, but hard seating gave more variation in shot-to-shot muzzle velocity than seating with the bullet distinctly but gently touching the powder. Very hard seating may even crush powder granules and change the charge's burning rate and the bullet's muzzle velocity to a material degree.
Target testing at 200 yards showed a 50 f.p.s. spread produced a 3/4" to 1" change in vertical point of impact. I'm sure that it would be magnified at 300 yards, where drop is greater due to longer time of flight. Any cleaning routine that left the bore free of fouling so that the bullet could be smoothly pressed down onto the powder seemed acceptable. Several ten-shot strings were chronographed with the bullet correctly seated with only the moisture of the bore changed after normal cleaning, and no excessive velocity variation was noted. Obviously, leaving a wet chamber area during loading will moisten powder and change velocities. Abnormal differences in patch thickness and width did produce velocity changes, perhaps due to changes in bore friction, and even gas cutting or leading if carried too far; but thickness variations up to .0005" did not appear important. This amount of thickness variation is often found even in the best patching paper and can be aggravated by humidity. Patch width did show that variations will influence velocity, so patches must be cut very carefully. Several brands of non-corrosive caps were tested, and while some were hotter than others, none made any real difference in muzzle velocity uniformity, except some caps not widely used by slug rifle shooters. I use the milder Dynamit Nobel 1075 non-corrosive caps, as they tend to give less disturbance during powder ignition. The CCI-Omark Caps are also excellent. With the 360-grain .40-caliber bullet, a two-grain slug weight variation gave no measurable velocity or target differences.
Since most of us can keep well cast slugs in this weight range, excluding rejects, there appears little more than psychological reason for weighing slugs into 1/2 grain groups. I prefer near perfection in bullets to ease my mind, but it's nice to know that an oddball a couple of grains off won't make too much of a difference.
Powder lot number was a mild surprise. Most of these tests were run using one particular lot of G.O.I. sifted FFg powder to keep things as uniform as possible throughout all shooting. All charges were scale weighed, not volume measured, to eliminate powder charge variation. About three-quarters of the way through the shooting, which was run in parallel with my .58 caliber Morris slug rifle, velocities jumped and stayed up for both rifles. This frustrating development was traced both to the accidental use of G.O.I. 2Fg powder of a different lot number and direct sunlight that heated both rifle and powder in the measure reservoir. Retesting of the two lots of G.O.I. did give a 20 f.p.s. to 30 f.p.s. muzzle velocity difference, but the hotter lot was equally uniform when tested and well within the normal lot-to-lot tolerance range of about three percent. The final testing using one lot of powder with bullets of the same weight and mild seating pressure kept the total spread of velocity over ten-shot strings to a remarkable 10 f.p.s. to 15 f.p.s. Testing with G.O.I. FFFg powder gave slightly higher velocities, but with more shot-to-shot variation, possibly due to increased chamber pressure.
These tests covered only the internal ballistics, not the other factors which can also cause jump or drop shots. Certainly they can also be the result of rapidly changing weather or light conditions, poor holding of the rifle, improperly designed bench and muzzle rest, badly styled stocks, or the inadvertent use of an extra thickness of paper patching when loading the rifle; but in these tests these factors were minimized and kept as uniform as possible. All shooting was conducted at the same range, at the same time of the day, and by the same shooter. One other noteworthy observation was that in running these tests in parallel with the .58 caliber rifle, the effects of variations in all these factors were relatively greater in the .40 caliber rifle than in the .58. For example, if the .40 used half the powder charge of the .58, a half-grain difference in .40 caliber powder charge made more difference in velocity than a one-grain change in the .58's charge. Seating pressure was similarly influential: much more important in the .40 than in the .58, and so on through all factors. But large-bore, heavy-weight rifles have compensating disadvantages like loading fatigue and controlling recoil over a string of shots and several matches.
Slug rifles are dependent on and somewhat limited ballistically by the use of unjacketed, patched lead or lead alloy bullets that are muzzleloaded and usually fired at muzzle velocities near the speed of sound, close to 1130 f.p.s. Bullet drag and wind deflection increase as velocities rise from about 900 f.p.s. and then pass through the sound barrier to 1500 f.p.s. Therefore, I wanted to tune this rifle for optimum accuracy and stability based on a muzzle velocity range around 1200 f.p.s. to 1300 f.p.s. to avoid the higher attainable velocity range around 1500 f.p.s. where wind deflection is relatively greatest. However, as the slug's muzzle velocity, initially over the speed of sound (Mach 1), drops back through and then below the sound barrier, the rearmost supersonic shockwave moves forward from behind to alongside and then ahead of the slug, forming part of the subsonic bow wave. This shock wave movement causes a momentary, random vibration and slight yawing of the slug. It's best to have this occur at a non-target distance so scores aren't possibly lower due to off-center shot holes. Chronrographing at 100 yards will tell if the bullets have dropped back through the sound barrier. Had the bullet shown instability with off-center shot holes, I would have adjusted its length if slight changes in powder charge didn't improve matters. If it showed yaw at 100 yards, but was stable at 200 yards, then the bullet would have been too short and needed a slight lengthening or drop in muzzle velocity. If stable at 100 and 200 yards but unstable at 300 yards, it would have needed a slight shortening or increase in muzzle velocity. Because we are limited to a fairly narrow muzzle velocity range and it sometimes takes more velocity change than desirable to change stability, we are left with the big job of making a new length of bullet, bullet molds (if not adjustable), and possibly swages to correct this problem. Often only a slight difference in length, as little as 1/16" in smaller calibers, is required if the rifling twist is close to proper. If too long, the bullet's nose can be shortened for preliminary testing by careful filing to reduce overall length.
Changing a bullet's length at the base end can involve many things because of the usual taper of the bullet's sides. A longer slug may be slightly larger in diameter, requiring a different powder charge, patch thickness and alloy. Too much shortening can reduce the bullet's diameter to the point of gas cutting if done from the base end.
If the bullet fit properly, had the correct length, was well loaded, flew stably, and its muzzle velocity was uniformly in the 1200 f.p.s. to 1350 f.p.s range but still grouped erratically, I'd have first made minor changes in powder charge or patch thickness. If it still grouped inconsistently in good shooting weather and was definitely not gas cutting, the bullet starter might not be correctly aligning the axis of the slug with the bore. A check for even length of rifling up the side of the bullet should have revealed any cockeyed seating caused by an off-center starter rod tip or loading rod tip. To gather these test-seated slugs it is best to examine recovered slugs from the oiled sawdust. Pushing them through only the false muzzle may not show the problem well enough, as the loading rod tip may be part of the cause. Comparative test firing by another skilled shooter might give a clue to the cause of the problem and is one of the best things to do when a good gun acts up. I'm indebted to Mr. Robert Sears, Associate Technical Editor of The American Rifleman magazine and Mr. Mac Chatfield, a well known shooter from the Minneapolis/St. Paul area, for their help on matters related to ballistics and bullet stability. Please refer to the Lyman Black Powder Handbook, First Edition (1975) for a very helpful discussion of muzzleloading ballistics.
It should be obvious by now that this tuning process isn't something that can be completed in couple weekends. Shrinking the group that last little bit or improving on long range ballistics is an invitation to weeks of hard work. Changing one item may produce the need for another compensating adjustment and a tail-chasing journey. Shooters with new rifles should get their rifles shooting as consistently as possible and then shoot them for a year or two, at least 500 to 1000 shots, before starting to change from their routine. Until we have the experience gained only from shooting our rifle frequently in the best and worst of conditions will we be successful.
If I may be permitted a personal note: Clark Frazier has coined the term Community Slug Gun, referring to all the help so freely given by other qualified people in his quest for accuracy with the Frazier Matchmate Slug Rifle he used to win second place in the 1978 Spring NMLRA Slug Aggregate. The entire process of tuning my .40 caliber Morris rifle was due not only to Bob Morris' skilled help and advice but also to the encouragement of Ernie Swain, Ron Wozny, Claude Roderick, Philip Orem, Bill Crowley, Larry Thompson, Ted Dunham, Eddie Ballotts, Jerry Simmons, Rex Haalck, Ferris Pindell, Richard Hoch, Walt Grote, and Peter Allan. We need more of this sort of cooperation. It makes our sport the satisfying challenge that our forefathers enjoyed.