Odd Accuracy Change

[P-990]

I was shooting my TC Encore today, getting ready for the MZ season (last minute practice ) and I suffered from what can only be described as an odd accuracy change.

About 15 shots into my session, with a target at 100 yards, my rifle went from 2 in the bullseye about 2" apart to dropping 2 shots low and right about 6", but also in a ~2" group. Nothing really changed that I could account for, other than a shift in light from bright and squinty to dark and dark.

My load was a 240gr XTP Mag Express and 95gr of Goex 2Fg ignited by a new Winchester T7 primer. After the first 2 at 100 I was feeling really good about this combo, but then with shots 3 and 4, I'm kind of worried.

Additionally, this rifle wears an aperture sight with a firesight on the front. Once MZ season is over, I'm giving TC a call and getting another front sight or two in differing heights to fine-tune my elevation. Right now the rear is topped out, which is fine as it WAS dead-on at 100. Just a detail.

First it's going to get a thorough scrubbing and maybe a return trip tomorrow before work.

I was just curious if anybody else has ever seen anything like this happen.

Thanks all.

[sundance44s]

You said 15 shots and it changed .. i`m guessing you were shooting hunting loads in the 100 to 150 gr of powder right ? if this is the case ... thats a lot of powder fouling and you need to clean the bore more often , during pratice with that many shots involved . After a good cleaning it should go back to the normal .

[arcticap]

It's the first shot that usually matters the most.

[P-990]

It's the first shot that usually matters the most.

Naw... I'll just carry an empty rifle for all of the effectiveness I am. Will neatly solve "Drat! It's a doe, it's a doe!!" or "Bummer, my gun is still on my lap..."

Quick range trip this morning after a thorough cleaning and I'll be happy.

[sundance44s]

It won`t take a serious cleaning at the range to keep up the accuracy at the range ..something like a spit patch or a little moose milk on a patch ...about every 4 /5 shots ..just be sure and pop a cap after the damp patch .

[Wild Bill Bucks]

P-990,

My first Thompson, quite a while back, had the same problem you discribe. When all was said and done, it turned out to be the mounting screw, on my back sights. I guess it had loosened, and was letting the sights move. Not a whole lot, but just enough to throw shots off.

I'm sure you have probably already thought about that, but thought I would mention it.

Shift in accuracy is a real bummer this close to season, hope your find out the problem.

[BigV]

I had a similar problem with my Stainless Steel TC Encore, but every 3rd or 4th shot would run astray. I shot this gun a lot and tried many combinations of bullets and powder and could never get consistency. I was shooting with a Leupold 3 X 9 scope at 100 yards. Finally out of frustration I sent the gun back to TC and they said nothing was wrong with the gun. I even tried a Leupold 2 X 7 I had sitting around and had the same results. I have since sold the gun a purchased a TC Omega with a laminated would thumbhole stock. I mounted the same Leopold 3 X 9 on the Omega and right off found it to be very consistent shot after shot. I was shooting 2” groups at 100 yards.

[cptmclark]

Yes, I've had it happen, and this year. I have a 70s vintage TC Hawken with a 90s vintage green mountain barrel. It has always shot inside two inches at a hundred (2 MOA) with my hunting load. Not this year. Changed scopes, tried several powder and charge weight and granulation changes (it didn't like musket caps or BP substitutes). I had been hunting for years with 120 FFG Goex and a 240 grain 44 bullet with the green sabot. (Guessing about 2000 fps). Better accuracy for me than the black "magnum" sabot with "magnum" 45 bullets. Lower charge weights gave minimal group improvement. Just recoil I think since it was so small. I had high hopes for the new pointy jobbies with high BCs, but nothing will shoot inside four moa except my old 240 xtp's and that is at 3 MOA. Changed scopes, checked mounts, tried different powders and powder charges, different nipples and caps. Nothing is better for me than Remington no 11s. The crown looks good and the bore is pristine. At least I have a 3 MOA hunter that's zeroed. Still, can't figure out where the accuracy went. Any suggestions will be gratefully accepted here too.

OH yea. this year the first shot from a clean dry wiped barrel is way off (2+") from the group that follows. Usually it's low and left. I clean (moist patch, two dry patches and one cap) between shots. I'm stumped. Like someone said, it's the first shot that counts. Any ideas?

[arcticap]

There's so many factors that could affect group size, not that I think that group size is the primary goal of a hunting load, but rather achieving a lethal shot with a high degree of certainty to hit within the kill zone is.
All anyone can do is to try to speculate.
Some folks have mentioned how minute changes in their wood stocks can affect barrel harmonics. I know it sounds strange because the barrels are so thick, but a little pressure or loose fit in the barrel bedding, or a loose fit between the tang & hooked breech, or forestock pressure from stock warpage/weather may affect accuracy. And that's also why some people weatherproof their stock's barrel channel with water soluble grease, apply stock wax to keep all moisture out of the wood or bed their barrels. This may only apply if a stock has been affected in some way, but not if it hasn't.
Another factor to consider may be sabot failure from the high pressures and temperatures resulting from higher powder charges and bullet weights. The plastic sabot may have a little bit too much stress being placed upon it to be absolutely consistent for every shot. Also, even though it's the same sabot you've always loaded over the years, maybe you just never realized their shortcomings.
Even scopes have built in parallax and reticle dimension factors which can limit accuracy potential. The reticle subtension dimension is how much of a target area is actually covered by the reticle at any given yardage and magnification. For example, if the reticle is covering a couple of inches of the target at 100 yards, then a shooter would literally need a machine rest to shoot any better than a 2 inch group since any movement would create a POA outside of the optimal 2 inch "reticle center" zone.
Different brands of sabots may also have different shooting characteristics. Does each & every individual sabot or sabot design always seperate from the bullet in the exact same place in the bullet's flight path and trajectory? Couldn't the timing of the bullet separation from the sabot affect accuracy? I recall that TC Break-O-Way Sabots are advertised as releasing from the bullet sooner after they exit the barrel to promote better accuracy. But do they actually work? I don't know since I haven't tried them yet. The new power belt type sabot designs have yet another base design.
Another factor to consider, whether it involves the first shot or group size is the barrel temperature. As the barrel heats up, the groups must be affected in some way since the barrel expands on the interior (where it touches the bullet) and exterior (where it touches the stock).
If achieving perfection was easy then it wouldn't be so much fun, right?

[cptmclark]

Yes, right.

[arcticap]

Did you ever examine the base of your sabot after firing? I wonder if all that Goex could melt or burn the base of the plastic sabot at all? I'm not sure if a wool wad, fiber wad or thin overpowder card (card stock) would be helpful for accuracy with a sabot or not, since the base might not expand the same. But the wads do often help the accuracy and consistency of lead conicals and patched balls, Might be worth a try whether the sabot is getting burned or not.
Come to think of it, the TC Break-O-Way sabots have a round washer shaped wool wad underneath them to hold the two halfs of the sabot together, and which helps to protect the base of the sabot and act like a gas check.
But their sabots do not have hollow "expansion" bases either.

[cptmclark]

Yes, the sabots look fine. I did check another thing and I hope I have found the problem. I checked the crown with a magnifying glass and it looks pretty bad. The bore is in great shape, but the lands are worn unevenly at the muzzle, probably from all the cleaning. Something to check, anyway. Thanks for the ideas.

[mcgiiver]

Articap said "I recall that TC Break-O-Way Sabots are advertised as releasing from the bullet sooner after they exit the barrel to promote better accuracy. But do they actually work? I don't know since I haven't tried them yet."

I just did a side by side test of T/C Breakaways vs, standard green sabots in my Lyman Cougar with Hornady 200gr XTP bullet (.429"). The standard green sabots shoot definitively better than the breakaway sabots. I finally got around to answering a question that has been nagging me.

[arcticap]

Thanks for the info., I've got a .54 Lyman Cougar but I hope to still give the TC Break-O-Way sabots a try.