The Jury Considers A TC .308

[Bitswap]

Hello everyone,

I thought I found the sweet spot for my Thompson Center 308 Pro Hunter and after testing the round a bit more I've come to realize that this TC doesn't appear to be as accurate as a bolt-action rifle. Here's what I'm seeing:

I'll put three rounds through the same hole at 100 yards, then the next shot will be 2" high. Then the next will be 1.5" high. The two ladder shots will be 0.25" within the horizontal, but vertical will be anywhere within 3".

So I look at my past targets to see if there's a history of this: Load development in 2 grain increments, testing clean and cold vs clean and dirty vs dirty and hot shots. And indeed the history shows the same thing, veritcal will be off within 3" while horizontal will be spot-on.

Had another shooter at the range check it out, he got lucky, only 1.5" on the vertical.

As far as I can tell, scope mounts are solid, tried two Leupold scopes so far, good rings.

Is this an inherent problem with the Thompson Center Pro Hunters? What should I try to solve this problem? Should I sell this nice little rifle and get a bolt action?

I've developed loads for my 308 Remington 700 PSS and Browning BLR and they didn't have this problem.

Any ideas would be helpful.

Thanks,
Bit

 

[Steve Koski]

Bit,

What kind of competitions are you shooting where this kind of accuracy is not satisfactory?

Have you tried different bullets?

Koski

 

[rbernie]

I had this happen a LOT with my Encores. I concluded that it's the way that the rifle bounces off the front rest, coupled with the non-free-floated front handguard.

My uber-heavy barrels didn't do this. My lighter ones did, but then they stopped when I laid sandbags on TOP of the barrel just forward of the scope bell to dampen things out.

I debated trying a floated handguard setup, but I wound up selling the whole gig instead in favor of a couple of Remington 700s.

 

[Grumulkin]

I think something is loose; most likely the scope mount since you say you've tried 2 different Leupold scopes. The mount doesn't have to be very loose to do this.

I've had quite a number of Encore barrels and there was only one that acted in any way like yours and that was a little different. My 30/06 barrel would put the first couple bullets high and to the left; it turned out to be a crown problem that was fixed by Thompson/Center.

For what it's worth, I bench shoot my Encores the usual way with the forend resting on a sandbag and I don't have the problem you describe. I'm not sure what kind of accuracy you're looking for. My scoped Encore barrels get from 0.5 to 1.5 inch 3 shot groups at 100 yards; is that good enough?

The group below was shot with an Encore handgun at 200 yards.

 

[Bitswap]

Thanks Rberine, you have confirmed my suspissons.

Steve, 3 moa in my book is unacceptable. Or would that be six... it always goes up 3 inches from the cross-hairs.

Grumulkin, I got a new Nikon bushmaster for this today. You could be right. If something were loose, like the scope mount, wouldn't that be horizontal as well as vertical on the errors? Going to have this scope 'professionally mounted' with locktight and hit the range again. Not holding my breath though.

I really believe it's the way the TC is set up... like a carriage gun that drops open. TC 'claims' 1 moa accuracy and this is far from it. I drive tacks with my hand loads in my 700 PSS and on a bad day get 1 moa at 200. Problem is that 700 weighs too much to take in the field.

I did try various combinations on rests: lead slead, the rock, bags.. all with the same results.

Time to give TC a call and see what they have to say about this, could be something wrong with the rifle.

Thanks again,
Bit

 

[jethro75]

I have the same problem eith my TC Encore I have tried just about eveything. Right now this is where I am at with mine, http://www.bellmtcs.com/store/index.php?cid=172 I havent completed this step yet so I cant tell you if it is a fix, but as soon as I do I can update. Hope this helps.

 

[Bitswap]

Good call Jethro.

But I've already tested for that in my load development with Redding competition shell holders that basically do the same thing. I tried bumping the shoulder 0.002 and 0.001 and the results are the same.

I do this for my rifle specific rounds. I aslo tried 'not' doing this and full-length sizing. As expected, full-length sizing was the worse, even worse then brand new brass... as expected. But the error was horizontal and vertical, not just vertical.

I also tried brass neck trimmed and non-neck trimmed with different tensions on the neck sizer.

About the only thing I haven't tried is seating depth. I'm seating 0.030" behind the lands which is where I usually start at. Federal Premium Match is about 0.050".

I'm not sure if a gunsmith or TC can determine the problem. I'm waiting to get a one-piece mounting ring before I try this again. Right now I have a weaver rail and mounts attached to that... too many connections.

If the vertical was on, this little gun would be EXCELLENT!!! Judging from the horizontal displacement, I should easily get 1/4 moa at 100 yards. There are many series of shots where the bullets just enlarge the hole and it happens quite often, then JUMP, up 2.5".

What's good enough for me? 1 moa would be fine (as advertised). And I'd get that using the cheapest Winchester brand new brass, by just reaming the primer pockets, deburring the flash holes and trimming to length.

Later,
Bit

 

[Bitswap]

I'm looking at this:

http://www.eabco.com/EncorePin.html

Think this will make a difference?

 

[Bitswap]

Sorry Jethro, I think you found the answer. I need to revisit my headspace adjustments, again, and try again.

From this site:

Likewise, cases that stick out too far a: interfere with complete lockup of the barrel in the frame, and b: put an inconsistent preload on the frame on a vertical plane that often rears its ugly head in the form of vertical stringing of shots.

Exactly the error I'm seeing...

Looking at my targets, the worse cases of "vertical stringing" were from the fire-formed brass that was just neck-sized... BINGO! Full length sized was off but vertically and horizontally... about 2 moa.

He's got a good method for checking this as well.

Thanks all,
Bit

 

[Phil A]

I had a similar problem with a Contender rifle barrel. Very accurate with the first set of shots until the barrel warmed up and it would start stringing vertically. This problem was due to a forearm hanger that was contacting the barrel between the screws. You may want to check the forearm contacts points and clearance. - Phil

 

[rbernie]

Forearm hangers (free-floated forearm tubes) are not standard; you have to buy a Bullberry or similar hanger.

 

[Bitswap]

Phill, I saw that was a problem on the H&H's and will make sure my forearm screws are torked alike.

We're not in Kansas anymore Todo.

These break-open actions re-define headspace. There is a measurement called "barrel to frame gap" that needs to be taken into consideration. Mine is 0.003". It is recommended that the headspace be 0.001" from the frame so my headspace needs to extend from the barrel by 0.002".

My mistake was I was taking the fire-formed headspace and bumping it back 0.001 to 0.002... not enough.

I guess bolt actions don't have this problem. I could get away not adjusting headspace with fire-formed brass a few reloads before doing anything about it: the indicator being it wouldn't chamber.

So I measure my 'pet' loads and come to find they're measuring about 0.033" beyond the recommended headspace. Makes perfect sence to me now. I'm putting undue pressure on the barrel when locked into firing position throwing the vertical off.

Note to self: don't use fire-formed brass in break-open actions without bumping the shoulder to 0.001 to the frame.

As a bonus, when I headspace correctly now, the rounds fit in my Giraud trimmer!!! Woot!!! no more hand trimming this stuff... major pain...

 

[Bitswap]

The jury is back and the judgement is...

I'm going to post a follow-up to this problem just in case someone searches for the same problem I was having.

I ended up reloading another batch in various configurations: Full Length Sized, Shoulder Bumb to TC, Neck Turned and not (just to see). I used my pet load of 43.7 grains of Varget (giving 2675 fps), 165 grain Sierra Gameking HPBT, CCI-BR2 primers, and cheap Winchester brass I got from Cabellas, neck sized using 0.331 bushing, and bullet seating depth to 2.750 tip to tip length (0.030 behind lands).

I should mention here that my usual brass prep includes: reaming primer pockets, deburring flash holes, trimming to length, etc, etc... you know the drill.

My best group was 5/8" moa at 100 yards, or 10 rounds through the same hole... well at least widening it a bit.

Here's a summary of the results:

1) A case with headspace too long will move your point of impact (POI) up from 1.5" to 2.5" and probably more depending on how far off it is.

2) A hot barrel will move your POI up about the same. Expect your barrel to get hot after four rounds of rapid fire (if that's possible with a TC).

3) Neck turning is definatly worth the effort.

4) Clean vs Dirty barrel doesn't make a difference. Heat is the key.

5) Groups were just under 3/4 moa at 100 yards using cheap brass and turning the necks, they were about 1.5 moa not turning the necks. Things really tighten up when using neck turned brass sized 0.001 to the breach.

In conclusion it is true what they say about the TC's accuracy. You just need to be aware of where things can go wrong. 1.5 moa using the cheapest brass isn't bad and an extra step, which should probably be done anyway, cuts that in half. Just keep your barrel cool.

Time for me to see what happens at 600 yards now using good brass...

Hope this helps,
Bit