Gunsmithing Heavy bolt lift on Bighorn TL3

[Mordamer]

I have been trying to troubleshoot a nuisance issue on my competition gun. I get heavy bolt lift intermittently and have not been able to link it to a cause. I can shoot some rounds and the bolt lift is so smooth it feels like there isn't even a round in the chamber. Then I have some rounds where I have to pull up with pretty good force to rotate the bolt. I have to use so much force that it throws me out of position and I sometimes have to grab the gun with my support hand to counter the rotation force. This is all with the same load and it is not a hot load. I thought this was a pressure issue at first and backed my load off 1 whole grain and still exact same issue. Here are the details:
Bighorn TL3
6.5x47 Lapua Chambered by PVA
Brass is virgin Lapua.
Primers are CCI small magnum
Bullet is 140 RDF seated .02" from lands.
Powder is 37.4 grains of Varget. Problem happened even when I loaded at 36.3 grains Varget. All powder done on a charge master. Load is accurate and SD was measured in single digits.

I have noticed that some rounds have very slight primer dimples around firing pin strike and some do not. I have not paid close enough attention to notice if these are the same rounds with heavy lift.

Any thoughts or suggestions helping me troubleshoot this issue are welcome.

 

[Supersubes]

On the fired cases with heavy bolt lift, have you tried to rechamber one of those? Is it heavy during the whole lift, or just when the bolt hits the extractor cam? Sounds like a case issue to me, not an action issue.

 

[z28kid]

I agree it kinda sounds like it could be related to brass, possible neck thickness issues or too long?
What is the total round count on the gun?

 

[Mordamer]

Just tried a few pieces of the once fired brass back in the chamber as suggested. They were heavy bolt close and heavy bolt open. It is heavy the entire bolt throw until I get to the extraction cam and once it gets to that point it feels pretty normal. Total round count on action is about 1000, but only 141 rounds on this barrel. I have a barrel I swop to for coyote hunting that is .223 AI and I just swap the bolt head. Never have any heavy bolt on the .223.

I am just about done firing my virgin brass. I have bumped the shoulders back on some of the once fired and they chamber with no resistance just like virgin brass.

 

[jonesturf]

Did you resize all the brass before loading or did you just load it and shoot? I'm guessing if you didn't once you provide the proper bump things will work out. I have the same action. Were any of them hard to close on when chambering as well?

 

[FALex]

Try bumping your shoulders .001-.002 OP, that's likely your issue.

 

[Mordamer]

That is what I am hoping is that they will be fine once resized, but they were all easy to chamber with virgin brass that was not resized.

 

[Sheldon N]

The thing I love about the TL-3 is how easy it is to check the "feel" of sized and fired brass. Pull the firing pin spring/shroud, clip a sized case under the extractor and try to chamber it. How easily does the bolt close? Is it falling freely with little to no effort at all on sized brass, if so then you've sized it enough.

Did you check how much headspace difference there was between virgin brass and fired brass?

 

[Mordamer]

I have not fired any resized brass through the gun yet. I will try it this week in preparation for a match on Saturday. I always check shoulder bump and aim for .002" bump. I am not having problems chambering rounds. I am having problems opening the bolt on a fired round.

 

[Sheldon N]

I have not fired any resized brass through the gun yet. I will try it this week in preparation for a match on Saturday. I always check shoulder bump and aim for .002" bump. I am not having problems chambering rounds. I am having problems opening the bolt on a fired round.

Was asking about headspace on virgin vs fired cases to find out if there was potentially a larger amount of headspace on first firing that was causing the pressure symptoms. Unlikely, just wanted to rule that out as a possibility.

 

[Supersubes]

Seems like you can rule out a receiver issue. Could it just be too hot a load? Heavy bolt lift through the whole stroke indicates a long case(headspace).

 

[LRI]

Take this with a grain of salt as powder companies are known to be on the conservative side of things. That said, you are +2.5 grains of the published maximum load. Might be worthy of a closer look.

http://www.hodgdonreloading.com/data/rifle

 

[Mordamer]

I shot a 10 round string over the Chrono last night. First 4 or 5 rounds the bolt lift felt completely normal. The last rounds of the string the bolt was heavy lift. All rounds were fired in less than 3 minutes. Accuracy was excellent and the SD of the 10 round string was 6 with an extreme spread of 17. I will go ahead and shoot the match this weekend with this load. I may have to find a different load later. When I reloaded some of the brass the primer pockets felt as tight as virgin brass.

 

[Zermatt Arms]

Seems like you guys are on the right track with this one but if she throws a curve ball hit us up.
Aaron

 

[Supersubes]

Seems like you guys are on the right track with this one but if she throws a curve ball hit us up.
Aaron

Good to see you here, Aaron. Thanks.

 

[Mordamer]

Was asking about headspace on virgin vs fired cases to find out if there was potentially a larger amount of headspace on first firing that was causing the pressure symptoms. Unlikely, just wanted to rule that out as a possibility.

I somehow missed this post. Brass headspace is .004" difference between virgin brass and once fired. This was in the realm of what I was expecting.

 

[Mordamer]

Thanks for everyone's responses. I wanted to ask the experts and make sure I wasn't missing anything obvious. I will rework my load with a different powder and start very low to make sure I am not just seeing pressure.

Bighorn, the TL3 action is the best there is. Thanks.

 

[rabeck27]

@Mordamer did you ever get this issue figured out? I’m having the exact same issue. My powder charge is 2.5 grains under max load and i get hard bolt lift after 4-5 rounds in a 10 round PRS stage.

Bighorn TL3
Proof 1:7 26” 6mm creedmoor
Virgin lapua brass
110 SMK .060 off the lands
2.875 COAL
39.1 gr RL16
REM 7.5 primer
2925 FPS


I’ve also tried H4350 and can’t get any more than about 2900 FPS without hard bolt lift.

 

[Mordamer]

@Mordamer did you ever get this issue figured out? I’m having the exact same issue. My powder charge is 2.5 grains under max load and i get hard bolt lift after 4-5 rounds in a 10 round PRS stage.

Bighorn TL3
Proof 1:7 26” 6mm creedmoor
Virgin lapua brass
110 SMK .060 off the lands
2.875 COAL
39.1 gr RL16
REM 7.5 primer
2925 FPS


I’ve also tried H4350 and can’t get any more than about 2900 FPS without hard bolt lift.

I am on about my 5th barrel since I had this problem. I think it may have had something to do with resizing lube left on the outside of my cases. I never really pinned it down to for sure cause. I have not experienced the problem in other barrels with normal pressure loads. The load I was using back then was on the hot end of loads. You should check for any and all causes of excessive pressure.

 

[LawnMM]

It's the brass, I've got about seven virgin pieces of Lapua 6x47L (necked down) that won't even chamber, going to have to bump the shoulders before the first firing, that's a first for me.

 

[rabeck27]

It's the brass, I've got about seven virgin pieces of Lapua 6x47L (necked down) that won't even chamber, going to have to bump the shoulders before the first firing, that's a first for me.

But mine chamber fine. I measured a .003 difference in headspace on virgin vs once fired which i would think is great, right? It’s only hard bolt lift after firing. Going in they are smooth as can be.

 

[LawnMM]

Ahh, missed that. If it's only after firing it's a pressure issue. Some barrels show it at different pressure points than others. If you're getting it post ignition that's probably the problem.

Try a grain under max and see if it goes away.

Varget is kinda fast for 6.5s most folks I know are using 4350

 

[rabeck27]

Ahh, missed that. If it's only after firing it's a pressure issue. Some barrels show it at different pressure points than others. If you're getting it post ignition that's probably the problem.

Try a grain under max and see if it goes away.

View attachment 7109401

Varget is kinda fast for 6.5s most folks I know are using 4350

This is a 6mm creedmoor and I’m 2.5 grains under max currently according to the Sierra manual for 110 SMK and RL16. Sierra manual lists max load at 41.6 grs of RL16 and I’m at 39.1. My velocity matches up pretty closely with the velocity in the Sierra manual as well. 2925 is slow for 6mm creedmoor. Ive also tried H4350 with roughly the same results. I’m leaning towards it being a virgin brass issue but that doesn’t make any sense to me. Why would virgin brass do that? The headspace isn’t too tight or excessive. I’ve run virgin lapua brass in my 308 for years with powder charges well over max load with no issues.

 

[padom]

This is a 6mm creedmoor and I’m 2.5 grains under max currently according to the Sierra manual for 110 SMK and RL16. Sierra manual lists max load at 41.6 grs of RL16 and I’m at 39.1. My velocity matches up pretty closely with the velocity in the Sierra manual as well. 2925 is slow for 6mm creedmoor. Ive also tried H4350 with roughly the same results. I’m leaning towards it being a virgin brass issue but that doesn’t make any sense to me. Why would virgin brass do that? The headspace isn’t too tight or excessive. I’ve run virgin lapua brass in my 308 for years with powder charges well over max load with no issues.

Every rifle is different, ignore the manual, its just a guide to start. Do your own load workup starting at say 37.5gr and work up to 40gr in .3gr increments. Every gun is different. Your 308 has no bearing on your current 6 creedmoor issue. Are you jammed or very close to the lands? It appears so if your COAL is 2.875" like you state. COAL's Im seeing in the Hornady manual are 2.81" for 110 A-Tips or less for other bullets.

Did you use an overall length gauge and find the distance to the lands for your bullet?

EDIT;

I do see the Sierra manual says COAL 2.875" but again, thats just a guide. What COAL does the 110smk hit the lands in your gun? You might have a shorter freebore and might be jammed explaining your pressure signs at lower charge weights......

 

[rabeck27]

Every rifle is different, ignore the manual, its just a guide to start. Do your own load workup starting at say 37.5gr and work up to 40gr in .3gr increments. Every gun is different. Your 308 has no bearing on your current 6 creedmoor issue. Are you jammed or very close to the lands? It appears so if your COAL is 2.875" like you state. COAL's Im seeing in the Hornady manual are 2.81" for 110 A-Tips or less for other bullets.

Did you use an overall length gauge and find the distance to the lands for your bullet?

EDIT;

I do see the Sierra manual says COAL 2.875" but again, thats just a guide. What COAL does the 110smk hit the lands in your gun? You might have a shorter freebore and might be jammed explaining your pressure signs at lower charge weights......

See previous post for more info on my gun. I’m .060 off the lands measured with hornady OAL gauge and bullet comparator measuring off the ogive.

 

[rabeck27]

Every rifle is different, ignore the manual, its just a guide to start. Do your own load workup starting at say 37.5gr and work up to 40gr in .3gr increments. Every gun is different. Your 308 has no bearing on your current 6 creedmoor issue. Are you jammed or very close to the lands? It appears so if your COAL is 2.875" like you state. COAL's Im seeing in the Hornady manual are 2.81" for 110 A-Tips or less for other bullets.

Did you use an overall length gauge and find the distance to the lands for your bullet?

EDIT;

I do see the Sierra manual says COAL 2.875" but again, thats just a guide. What COAL does the 110smk hit the lands in your gun? You might have a shorter freebore and might be jammed explaining your pressure signs at lower charge weights......

And i tried anywhere from .010 off to .080 off. I settled on .060 because accuracy was best. I did do load workup in .2 increments from 38.0. I’m just surprised i can only get 2925 out of it. Seems awfully low so i thought maybe there was something else I’m missing. The weird thing is that it doesn’t give me hard bolt lift every time and i even tried backing down to 38.x (i don’t remember exactly what it was) but still got random hard bolt lift. So it doesn’t seem like I’m right on the edge of max load for this rifle.

 

[ugsly308]

Do you have a bore scope to look and see if you have a carbon build up in the neck or throat? My brother and I's 6 creedmoors running H4350 build a carbon ring and will creep up our velocities and pressures. It was to the point we would blow a primer or two in a 100 round match and couldnt figure it out.

 

[rabeck27]

Do you have a bore scope to look and see if you have a carbon build up in the neck or throat? My brother and I's 6 creedmoors running H4350 build a carbon ring and will creep up our velocities and pressures. It was to the point we would blow a primer or two in a 100 round match and couldnt figure it out.

Hmm yes i do. I’ll check that out. Thanks