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Heavy bolt lift related to bore diameter?

Jody Simcox

Jody Simcox

Recently received a new 6GT that I purchased for my father’s 75th- he’s never owned a centerfire, but I know he would enjoy coming to the farm with his grandsons and shooting some steel. I bought 100 rounds from Copper Creek and 100 pieces of Alpha brass, so he could shoot without waiting on me. I thought I would do some seating depth testing while I broke in the barrel.

Defiance Tenacity
Muller Works Sendero in 6GT
(23”)

I loaded 50 rounds with 32.5 Varget, Berger 108 Elite Hunters, and CCI 450’s. Max charge is 34 gr per Hodgdon. Seated from .020 off to .120 off

Cleaned barrel after each shot x 20, then after every 3rd shot x 10.

Had heavy bolt lift after all 50 shots. No flattened primers, no cratering. MV 2725.

Someone said the heavy bolt lift was because my bore diameter was.236 and not .237...I didn’t know about bore diameter when I ordered my gun...I just asked for a 6GT....specified barrel length and cerakote color only.

this gun is no fun to shoot with such heavy bolt lift...you have to stand up after every shot to eject and re-cycle...

What steps should I take/try next?

thanks
 
Barrel dimensions have a large effect on pressure, but bore diameter is way down the list on what matters. My bet is the new brass that does not fit the chamber and if that 108 is similar to the regular 108 Berger they tend to build pressure anyhow. You may also want to verify even lug contact. It very well could come down to barrel dimensions, but the .236 bore is not the problem.
 
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Virgin Alpha brass...even the loaded rounds (with same bullets) had heavy bolt lift...GG is who said the .236 was the problem...said he’s posted it in multiple places, but I can’t find it...any suggestions regarding different bullet? Some have suggested slower powder and/or non-magnum primers? Send the gun back to smith? I’ll talk to them today...
 
I had the same problem with a 30x47 I had to turn the necks down to 0.010, the "click" and hard bolt lift went away.
 
My method for finding case fit is for making sure your sizing die fits the chamber. Neck clearance is another topic. You should have been given a reamer print or informed about neck turning to prevent a potentially dangerous situation. But that does bring up a good point, the OP need to verify neck clearance.
 
Alex I'm not saying your method doesn't work. What I'm saying is it didn't define my problem. It helped me find my headspace for bolt fall/lift but only after I found out I needed to turn the necks down.
I never got a reamer print, gun was bought second hand.
 
Jody Simcox said:
Recently received a new 6GT that I purchased for my father’s 75th- he’s never owned a centerfire, but I know he would enjoy coming to the farm with his grandsons and shooting some steel. I bought 100 rounds from Copper Creek and 100 pieces of Alpha brass, so he could shoot without waiting on me. I thought I would do some seating depth testing while I broke in the barrel.

Defiance Tenacity
Muller Works Sendero in 6GT
(23”)

I loaded 50 rounds with 32.5 Varget, Berger 108 Elite Hunters, and CCI 450’s. Max charge is 34 gr per Hodgdon. Seated from .020 off to .120 off

Cleaned barrel after each shot x 20, then after every 3rd shot x 10.

Had heavy bolt lift after all 50 shots. No flattened primers, no cratering. MV 2725.

Someone said the heavy bolt lift was because my bore diameter was.236 and not .237...I didn’t know about bore diameter when I ordered my gun...I just asked for a 6GT....specified barrel length and cerakote color only.

this gun is no fun to shoot with such heavy bolt lift...you have to stand up after every shot to eject and re-cycle...

What steps should I take/try next?

thanks
Click to expand...
Draw a diagram of the brass before and after ignition & record all the dimensions as the sample I’ve posted to help analyze
 

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I'm hesitant to even post this, but here it goes.

I had a 6GT barrel spun up for my F-T/R gun while COVID had the matches shut down, and it happened to me too. Exact same scenario; 107s at 2900 out of a 30" barrel. I got heavy bolt lift with no other pressure signs.

My brass was Alpha brass I had neck turned to .012 (removed ~.002 from the necks).

After the first firing on the brass, the issue never returned, and I've run the bullets to much higher velocities without any pressure issues.

I subsequently switched over to Hornady brass, and had it happen again on the first firing, but it wasn't as bad.

If you read the 6GT facebook page others have posted this too; George is on there telling them they're doing something wrong. I can kinda see his point too, I see some super questionable reloading practices posted there, but I can tell you in my case it wasn't a factor.

I can't explain why it happened, but I can tell you what wasn't a factor for me:

1.) Moisture in chamber
2.) Headspace
3.) Neck diameter (cases came out .272)
4.) Bore diameter (.237 on my bart)
5.) Charge weight (was on the low end of the spectrum, and have since fired much higher weights).
6.) Depth - Was ~.020 off the lands.

Edit: I have nothing to back this up; this is merely a wild accusation, but I half way think there's something screwy with the reamer spec they've got.
 
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Jody Simcox said:
Loaded rounds consistently measure .2695”....
Click to expand...

That's pretty snug but would be ok if everything were perfect. Sometimes, the neck portion of the chamber has some taper in it.

If were at my place, I'd take a single case, verify that the shoulder was getting pushed back .002 after sizing then turn the neck so it measured around .265-.266 with the bullet seated and see how it acted.
 
.265 with a seated bullet seems like a lot of neck clearance to me in a .273 neck (at least according to the reamer print)....but...maybe the reamer no longer matches the reamer print?? Possible?
 
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