Bolt Lift Early - Lower than expected velocities

[TikkaSporter]

I'm working with a new rifle build and something seems amiss...

Info on the rifle:
- Bartlein Barrel, 5R, 22", 1:8.2 twist
- 260 Remington
- Defiance Deviant Action
- Chambered by GA Precision
- 0.088" freebore (I think)
- Load: 140 gr Berger Hybrids, H4350, 0.020" jump to lands, 2.880" COAL, Fed210M primers, Lapua 260 Rem brass

When working up loads I started to get bolt lift at 39 - 40 grains of H4350 which seems to be really low for this combination. At these powder charges I'm getting about 2500 - 2600 fps. By bolt lift I mean the bolt opens fine/normal until the last 1/4 inch of movement, then I get resistance or stiffness. As the charge goes up, it eventually gets a little stiffer... But there are no other pressure signs all the way up to 42.6 grains (didn't try anything above that). No ejector marks, not even faint ones, no primer cratering. Maybe a little bit of primer flattening as the charge went up. This even happens with virgin brass. I'm having trouble believing this is just a pressure issue, hence running it up to 42.6 gr in 0.4gr increments to see if ejector marks or primer issues would eventually show up... They didn't.

At about 41.5 grain H4350 I'm getting just over 2700 fps and decent accuracy... But enough bolt lift that it's bothersome.

The things I've checked for or done so far:

- No carbon build up in the throat freebore area
- lubed the 'cocking cam' and back of the lugs
- Happening with virgin brass so I doubt it's donut issues
- Barrel should be broken in by now at about 120 rounds... Followed the standard break in procedure
- Tried a different powder (RE-17 because I had some laying around)... Had the same bolt lift earlier than expected (37 grains) and even lower velocities.
- Ran the Virginia brass through a FL sizing die (Redding Type S)
- Bumped the shoulder back 0.002" - 0.003" on fired brass

So I guess the questions are;

- is it normal to get bolt lift pressure signs well before ejector marks or cratered primers?
- Could there be something wrong with the bolt/action/barrel causing this?
- Is this really an over-pressure issue?
- Do I really need to back my loads all the way down to where there is no increased bolt lift, running 140 gr bullets at only 2500 - 2600 fps? Ugh...
- What else can I do/try?

From everything I've read it seems like I should be able to get up to at least 42.0 grains of H4350 and 2750 fps without any pressure issues with this action/barrel/caliber. Am I wrong?

 

[Joe Salt]

Are you turning the necks on the new brass. Even with a no turn neck, I still like about .003 clearance!

Joe Salt

 

[Dans40X]

How are your groups?

Accuracy & doping skills will trump velocity!

 

[Yellow11]

Just a stab in the dark, but check that the bolts attaching your Piccatiny rail are not too long and protruding into the action and catching the bolt.

 

[jlow]

Does seem strange. With my AI running 26” Bartlein barrel, same components except primer (Tula), I am getting around 2,850 fps with 43.3 grains of H4350 with no pressure signs or bolt lift problem. Can you load and extract a dummy round with no problem? i.e. do you get the bolt lift problem if the round was not fired?

 

[TikkaSporter]

Are you turning the necks on the new brass. Even with a no turn neck, I still like about .003 clearance!

Joe Salt

Haven't turned any necks yet. Pretty sure I have 0.003"+ of clearance without turning.

 

[TikkaSporter]

Just a stab in the dark, but check that the bolts attaching your Piccatiny rail are not too long and protruding into the action and catching the bolt.

The Picatinny rail is integral (machined into the action) and there are no bolts for it. But that said, the front action screw contacted a bolt lug when I tried torquing it a little. I could feel the bolt starting to bind after I tightened the front action screw. I didn't force anything, backed it off, and used some emery cloth to grind the end of the screw down until it didn't protrude into the action/lugs

 

[TikkaSporter]

How are your groups?

Accuracy & doping skills will trump velocity!

Around 0.35" - 0.5" (@100 yards) in the 41.0 - 42.0 gr of powder range... Just have some bolt lift there

 

[expiper]

OP ,,,what is the dim. of the reamer/chamber at the .200" line,,,it sounds too tight compared to the ctg brass that you are using,,,,it should be .003 larger than new/unfired brass,,,,Roger

 

[ShootDots]

I think your biggest problem is your freebore... I have a .260A.I. that shoots 140 Berger LRBT's great with 43.8grs of H4350. There is absolutely NO pressure signs / problems whatsoever. BUT I have a .168Freebore, literally almost DOUBLE the length of your freebore. You are compacting that powder by having to seat the bullet well into the case shoulder area. On top of that you are jumping your bullet 20K. This is shoving the bullet even deeper into your case. Hence the low charge weights and what appears to be pressure signs.

SOLUTION: Go to your 'smith and have him take one of those "unithroaters" and open that freebore up about 85-100K and you will be fine. You will have to start load development all over again, however, your problems should vanish..

 

[4xforfun]

Have you checked the accuracy of the chronograph?