7mm08 Chamber Question

TAstro · May 11, 2020

Question for you all.
This is in regards to a Thompson center compass in 7 mm 08. I have not been able to get this thing to shoot groups for anything (I’m talking 6-10 inches sometimes..). I’ve tried a bunch of different brands of ammo, as well as a wide range of weights. Now, I wanted to test the chamber, because I have checked everything else on the gun including swapping scopes and rings, pulling the action, etc. I wanted to check the chamber because I heard another person with a Compass had a very badly cut chamber.. With a $250 rifle, who would be surprised.? Now I will jump into the details and relevant facts.

I wanted to test the jump of the bullet to the lands vs the factory ammo. I had a box of 139 gr Hornady sst ammo, half shot. I took a case (shot out of this rifle) dented the mouth a little to retain the bullet, colored the bullet with a sharpie, and seated it in the brass. I then chambered the round gently and got a COAL measurement of 2.8945 after the round was pushed back into the case as it contacted the lands. I then measured the factory SST round and got 2.7670. . I added a little more tension to the neck of the fired brass, colored the bullet, and ran the test again, and got 2.8950 (so fairly consistent).
The difference in factory COAL (2.7670) and the round I colored and pushed the ogive to lands was about .2185 inches. Does this seem like the chamber might be cut a little funky? Does anybody have any other suggestions that I should take a look at? This is pretty much the last thing I can think to look at before I print this shipping label and send it back to Smith & Wesson.

I am pretty much entirely unexperienced gunsmithing and evaluating a chamber, so any feedback or advice is a big help.

Also I can’t reload the round that long, won’t fit in mag.

TAstro · May 11, 2020

I guess the proper question would be (I think);

Is .2185 too much free bore ?

Could this be cause for terrible accuracy?

Doco Overboard · May 11, 2020

Ok so a couple things. The area just ahead of the chamber mouth is the leade and not the chamber. The two are interrelated but not exactly the same thing. The best way to determine a chamber measurement is to do a cast using cerro safe. By doing that you will have a snap shot so to speak that will provide accurate measurements. Usually a little bit of the leade is captured along with the cast which will allow you to determine if the bore of the barrel is not concentric with the bore of the chamber or if there is a radical departure that can be observed through careful measurements or even b eye in an extreme case.

Having brass available to you fired from the same rifle is valuable so it can also be inspected to determine if there are any apparent abnormalities. Crooked necks, bulged cartridge bases, wildly expanded case mouths, poor extraction etc that would point to a chamber problem.

Another thing you can do is peer into the chamber from the rear of the rifle and have a look at the chamber and leade in as best you can and see if you can see some obvious defects. A heavy circular ring right where the rifling starts,missing or defective rifling and then through the bore in brite light and see if you can see a dark spot that doesn't go away no matter how you tilt the rifle. Then at the opposite end of the barrel at the crown checking for defects the same way.

I wouldn't discount a problem as described with your rifle but I would look at other things that could contribute to the problem before I went through the trouble of sending it back. You may find something you over looked like a problem with the way its stocked up, poor bedding at the receiver or action screws that are loose or not fastened properly.

Sometimes when you get frustrated its easy to overlook something you ordinarily would have detected.

Doco Overboard · May 11, 2020

TAstro said:
I guess the proper question would be (I think);

Is .2185 too much free bore ?

Could this be cause for terrible accuracy?

No, certain rifles load combo's can respond differently but if you ever looked into a Mauser or Weatherby rifle you may say I paid for a whole barrel not 3/4's of one. (sarcasm intended).

You could be getting messed up pinching a case shut just enough to hold the bullet. It could be dragging after being imprinted on the rifle a small bit when you withdraw the bolt.

Use a an unfired case or one with that's got slots cut into but has been resized or selected to hold the bullet correctly.

John from MD · May 11, 2020

Buy some Cerrosafe from Brownells Inc. It melts around 190* (AFAIR) and you can pour it into your chamber to make a chamber cast. You really only need to block about a half inch past the lead and you don't have to make the whole case, just do about half. By doing only half of the case, you will lessen the chance of filling the locking lug area making removal difficult. Cerrosafing is easy and there are numerous tutorials on line to show how it is done. That is about the cheapest way to take a look at your chamber and lead to see if there is a problem.

E.Shell · May 11, 2020

Doco Overboard said:
...You could be getting messed up pinching a case shut just enough to hold the bullet. It could be dragging after being imprinted on the rifle a small bit when you withdraw the bolt.

This happens most of the time if the case neck is not sized to actually support the bullet. The tapered surfaces mating (tapered cone ogive vs tapered cone leade) have soft contact but high engagement.

Use a an unfired case or one with that's got slots cut into but has been resized or selected to hold the bullet correctly.

Yes, this is better.

The method I have taught and used reliably is to seat the bullet long in a properly sized case and try closing the bolt. It should not close. Seat the bullet deeper into the case until the bolt will just barely close with no 'feel' of bullet engagement.

At this point, it's probably still touching, but cannot be detected above bolt friction. Leaving the seating die set where it is, get a new bullet and case and assemble another test cartridge. Before chambering, very lightly brush the bullet bearing surface with #0000 steel wool by TWISTING it. The object is to have very light marks *around* the bullet - in the opposite direction the rifling will engage.

Carefully/gently close the bolt on this round, and then carefully remove it and note where the rifling marks are, and about how long. You should see the light scratches from the steel wool, and the rifling contact points will be glaringly obvious because they will hit the scratches at right angles.

You can use the length if the rifling marks to judge how far the seating die must be adjusted. Continue to advance the seating stem and re-polishing the bullet with the steel wool until you no longer see the rifling contact.

Assemble yet another new test cartridge, using your last seating die adjustment and try it in the rifle. You will probably see the smallest bit of contact to tune out and once this is done, you're at 'zero'. Then add your planned clearance to your seating depth and you're set.

Much harder to explain than to do, and provides very reliable results.

KRC · May 12, 2020

How long can you load the 139gr. SST's? Try shooting as long a COAL as you can load (single feed - no magazine)? Try another heavier bullet that you can seat further out? 154 gr. SST? Does this improve precision? What are the SAAMI specs for that chambering?

KRC · May 12, 2020

Also, a Teslong borescope ($49.99 Amazon Prime) can be a valuable tool for bore and chamber inspection.

TAstro · May 13, 2020

After checking everything out, I think I’m just gonna send her back to Smith & Wesson. Something is definitely not right somewhere, beyond my expertise

Biggfoot44 · May 13, 2020

If something is consistently giving 6-10 inches, my first thought is the muzzle crown .

gwchem · May 13, 2020

Biggfoot44 said:
If something is consistently giving 6-10 inches, my first thought is the muzzle crown .

Either that, or the scope is mounted with scotch tape.

Uncle Duke · May 13, 2020

gwchem said:
Either that, or the scope is mounted with scotch tape.

Or action screws that are loose as a goose.

TAstro · May 13, 2020

I’ve torqued the action screws. I’ve pulled the action and checked for anything obviously wacky, didn’t see anything. Rings, bases, all torqued to spec (with a torque wrench). Crown is flawless (at least it is the same as it left the factory).

Ahe is going back to S&W. Hopefully they will tell me what was wrong and not just send me a new rifle.

TAstro · May 13, 2020

And, I’ll loan the rifle with a couple boxes of ammo to a known member in good standing if any of you want a crack at it.. it’s one of those things where I just want to know what is wrong.!

John from MD · May 13, 2020

Many years ago I bought a Savage Long Range Heavy Barrel target rifle in 300 Win Mag. From the first shots I new something wasn't right as I was getting shotgun patters at 100 yards. I brought it in to 50 and it still didn't group. When I ran out of things to check, I decided to check the barrel twist. Surprised the hell out of me that it was 1 in 14 instead of 1 in 10. Sent it back and Savage sent me a new tack driver.

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