Tuesday, July 5, 2011
Tighten Up Sabot-Bullet Fit For Tighter Groups
Early rifle makers of the 1700s, and to some degree into the early to mid 1800s, often produced their own barrels - generally out of necessity. First, they forged a long and narrow ribbon of steel, and hammer forged that around a mandrel of sorts, keeping the metal as close to white hot as possible, literally welding the wraps with every strike of the hammer. Once the barrel blank was finished, the bore was reamed and polished, then it was ready to be rifled. Except for the hand welding of the steel, most of this arduous work was done on a wooden rifling bench. And while the riflesmith likely started out to produce a certain caliber, that was likely rarely achieved. Say the bore was to be a ".45"...and in the end was more like a ".46" or ".47" caliber. But, that was not a big deal in those days, because once the barrel was done, the maker generally made a bullet mould that would produce the proper diameter ball or bullet for that barrel.
Fortunately, with today's advanced machining and mass production capabilities, it is now far easier to produce bore sizes that are far more precise. Still, that does not mean that all bores of a given caliber share exact internal measurements. Take the popular .50 caliber sabot-shooting in-line rifle bores, some of which have been as tight at .499", and some going to as large as .504"-.505" on the other end of the spectrum. Finding the optimum sabot-bullet fit for a particular bore has proven extremely frustrating for shooters who demand tight downrange groups.
Even the barrels on rifles of the same make and model, produced on the same machinery, can typically have bores that vary .001" to .002" from barrel to barrel, or from one barrel run to the next. And this is often due to the slight wear of the boring, polishing, and groove cutting/forming tools used to produce the bore. Even this slight variation can affect not only the accuracy of the rifle, but with some of today's newer and harder to ignite powders, like Blackhorn 209, it can also affect the consistency of igntion - especially when a sabot-bullet combo is extremely loose fitting in the bore.
Earlier this year, a shooter from Vermont contacted me about using a wrap of Scotch-type tape around the .452" Hornady XTP jacketed hollow-points that he was loading and shooting with his .50 caliber rifle. He claimed that with the black .50x.45 sabot that came pre-packaged with the bullet, from Hornady, the rifle was just way too tough to load...and when he went to the easier-loading Harvester Muzzleloading black .50x.45 Crush Rib Sabot, the combination did not give enough compression of the Blackhorn 209 charges he wanted to shoot. So, he simply put a wrap of clear celephane tape around the bullet, producing a slightly tighter fit when the combo was pushed down onto the powder charge with the ramrod. The added compression produced spontaneous ignition...and better accuracy.
Personally, I wasn't too crazy about the idea of putting adhesive tape onto bullets...so I decided to do some experimenting with the non-adhesive thin white Teflon plumbing/thread tape. And I had just the rifle for the testing.
Since February 2009, I've been shooting a prototype of a rifle which the new Knight Rifles company has introduced as their bare-primer version of the DISC Extreme - the "Mountaineer". Early on, I found that this particular rifle had a bore running right at .502 from land-to-land. When I first shot in cold weather, with temps in the 20s and 30s, the 300-grain .451" diameter Scorpion PT Gold and the standard black .50x.45 Crush Rib Sabot loaded just tight enough to give the compression needed for spontaneous ignition with the hefty Blackhorn 209 charges both the rifle and I liked. However, as soon as the weather warmed to the point in late spring/early summer when the average temperature during my range sessions was getting into the 60s...I noticed that the combo was loading way too easy, and my accuracy was beginning to wane. So, I switched to the tighter fitting red .50x.45 Crush Rib Sabot, and the Mountaineer prototype soon was punching the 300-grain Scorpion PT Gold in tight sub 1-inch groups again.
So, with a fresh roll of the white Teflon tape in my shooting box, I headed for the range on an overcast afternoon (with a few light showers thrown in) with the temperature right about 70 degrees. I shot the rifle a few times with the red Crush Rib Sabot, and 2 of the 3 groups shot were inside of an inch. I gave the rifle plenty of time to cool between loading and shooting again. But when I made the switch to the looser fitting black .50x.45 sabot, I experienced two hang fires out of the first three shots. It was very evident that in the hotter weather, the .451 Scorpion PT Gold and the black Crush Rib Sabot were not all that compatible out of this particular rifle bore.
I had already determined that it took about 1 1/2 inches of the Teflon tape to encircle the 300-grain bullet once...so I pulled out and measured a strip just 1/8th inch over 3 inches in length - enough for two wraps. The idea behind using a non-adhesive type tape was to have the spin of the bullet shed the Teflon shim instantly as the bullet left the muzzle. So I laid the strip of Teflon on the knee area of my blue jeans...and rolled bullet across from right to left. The Knight/Green Mountain barrel of the Mountaineer has a "Right Hand Twist"...meaning the rifling rotates clockwise. When the bullet and sabot leave the bore and separate, the loose end of the Teflon would catch the air and be instantly peeled from the bullet...rotating at extremely high rpm's in the same clockwise spin.
Anyway, that was my theory.
And it must have been a sound theory. The first three shots were spontaneous, and printed right at 1 1/4 inches. Two more groups were right there with them. One of the showers that afternoon had come and gone, and the temperature had dropped to about 60 degrees...so I went back to shooting the bullet and black sabot WITHOUT the two wraps of Teflon. The first attempt just sort of spit the bullet out. Loading the rifle again in the same manner, WITHOUT the Teflon tape, the rifle fired okay for the second shot, but was about 4 inches higher than where the groups shot WITH the two wraps had printed. But the third shot experienced a lengthy hangfire, and didn't even print on the paper target at 100 yards.
I decided to add another wrap, tightening up the fit just a bit more...cutting a strip that was right at 4 1/2 inches in length. I put a small dot on the bullet just above where the wrap began, and when I reached the other end, found that the Teflon encircled the bullet approximately 3 1/4 times. When first experimenting with rolling the tape around these bullets, to get everything to sit nicely down into the cup of the sabot required that about 1/8-inch of the 1/2-inch wide tape extend below the base of the bullet as it is wrapped around. Then after the loose end is rubbed down, sticking to the wrap below it, the "skirt" can be folded toward the bottom-center of the bullet. This prevents the thin Teflon from riding up when the wrapped bullet is inserted into the sabot cup.
So, how did the bullet shoot with the additional wrap? Two of the three groups shot, using 110-grains of Blckhorn 209, stayed well inside of an inch...the other was right at an inch. The best group of the three measured right at .660" center-to-center.
All of this testing was done with the very light, very thin white Teflon. When a single thickness is compressed between the jaws of my calipers, the material measures just over .001". The heavier duty pink Teflon, compressed in the same manner measures just shy of .004". Next time I head to the range, one morning later this week, I'll give two wraps of the pink non-adhesive tape a try. I'll post an update.
If any of you have experienced ignition or accuracy problems due to a loose fitting sabot-bullet combination, you might want to give this a try. If you do, be sure to come back here and leave a comment on how it worked for you. - Toby Bridges, NORTH AMERICAN MUZZLELOADER HUNTING
NOTE: All NORTH AMERICAN MUZZLELOADER HUNTING primary test rifles are topped with one of the excellent 3-9x40mm multi-reticle Leatherwood/Hi-Lux Optics muzzleloader hunting scopes. If you are in the market for a new scope this season, be sure to go to the link below...and when you click on "ENTER", look for the special pricing offer near the top of the opening page.