6BRA sizing vs heavy bolt lift

[Sheldon N]

The recent thread on 6BRA dies and base sizing got me thinking about this issue. I know that if you don't have sufficient base sizing on your dies that you can end up with clickers on bolt lift. Been down that road with a small chambered Dasher before.

However, what about just overall bolt lift effort when you are getting closer to max pressure? Will sizing more at the base of the case give you a bit more leeway before you start to see heavy bolt lift? Or what about slightly more shoulder bump (ie. 0.003 bump instead of 0.002), will that also help ward off premature heavy bolt lift?

I'm looking for ways to help make the bolt run more smoothly, am already running a fairly conservative powder charge and velocity.

 

[MDM]

Here's something I found interesting, but not sure if it even correlates to the situation... As I was forming brass on my first bra, I was shooting over the magneto, just various charge weights from 28.5-30 varget. I was really impressed that the numbers were all good, no matter the charge, biggest spread I had in any forming loads was 14 fps. What I found interesting was, say a 30g varget load ran at 2800 during fire forming, then I put 30g in a fully formed case, same exact speed at 2800. Interesting because there is always the thought out there that forming your brass will rob a little pressure and velocity, even in non improved calibers. The opposite of what I saw running that test. So, based on that, I'm not sure any tiny variance in case dimension is going to yield any quantifying result in change of perceived pressure/bolt lift issue. Just a thought. I know you are running in PRS/field type matches, same as me and I understand the need to keep it running smoothly. My load is running a paltry 2905, but it runs easy and is accurate as hell. My second barrel I'm just now getting going is slower yet, but still too early of a round count to tell. I'll let you know if my die change has any effect on pressure/speed as soon as my bushings get here.

 

[Sheldon N]

So, based on that, I'm not sure any tiny variance in case dimension is going to yield any quantifying result in change of perceived pressure/bolt lift issue.

Thanks for the reply. My thinking is that if the case starts out smaller due to sizing, that it's more likely to spring back from the chamber walls and extract without effort. Would love for the bolt to run just as easy during live fire as it does when dry firing. Most recent load is 2923 fps with a 105 out of a 26" barrel, but had some rounds that were a bit heavier on the bolt lift than I'd like. Not sure if that's pressure or sizing related, no other pressure signs like ejector marks or flattened primers or anything.

 

[steve123]

Thanks for the reply. My thinking is that if the case starts out smaller due to sizing, that it's more likely to spring back from the chamber walls and extract without effort. Would love for the bolt to run just as easy during live fire as it does when dry firing. Most recent load is 2923 fps with a 105 out of a 26" barrel, but had some rounds that were a bit heavier on the bolt lift than I'd like. Not sure if that's pressure or sizing related, no other pressure signs like ejector marks or flattened primers or anything.

Have you used the same brass in different chambers/barrels?

I have my own 6x47L reamer which has been used for 7 barrels, and I use the same brass still. I also have a custom die which was made for the 2cnd barrels chamber.

I know the chambers have been slightly different each time the GS did his own chambering. The result is some brass, although FL sized, gives a stiffer bolt lift, most don't. I've been tossing the brass that gives the stiffer bolt lift as I shoot it. All the brass will chamber like normal after being sized but when fired, not so much.

Some of my brass has had over 38 firings on them but pockets hold a primer well so I keep using them.

 

[Sheldon N]

Thanks Steve, that's good info. For sure brass has a memory. I've got some cases from a match that was shot in the rain with pressure and those have always stayed a little larger than others.

Out of curiosity, did you measure base diameter on the ones that had stiffer bolt lift? Or perhaps it was headspace/shoulder expansion on firing?

 

[Delfuego]

Sheldon, I think our boys are running 3-4/1000 if I recall. They said it helped everything. I might be talking out of turn, maybe @moondog will see this and chime in.

 

[Curious]

Sheldon, I think our boys are running 3-4/1000 if I recall. They said it helped everything. I might be talking out of turn, maybe @moondog will see this and chime in.

In what area?

.200 line or headspace?

 

[moondog]

Sheldon, I think our boys are running 3-4/1000 if I recall. They said it helped everything. I might be talking out of turn, maybe @moondog will see this and chime in.

I had the opposite, hard bolt close on fire formed brass. I bump it .0045, problem gone and super accurate.
I would ck the .200 line on fired vs sized.

 

[Sheldon N]

I had the opposite, hard bolt close on fire formed brass. I bump it .0045, problem gone and super accurate.
I would ck the .200 line on fired vs sized.

For me bumping the shoulder 0.002 or 0.0025 gives me zero resistance bolt close, I'd need to shorten my die if I wanted to bump the shoulder further. At the 0.200 line I'm getting about 0.0006 or 0.0007 sizing from fired brass.

 

[steve123]

Thanks Steve, that's good info. For sure brass has a memory. I've got some cases from a match that was shot in the rain with pressure and those have always stayed a little larger than others.

Out of curiosity, did you measure base diameter on the ones that had stiffer bolt lift? Or perhaps it was headspace/shoulder expansion on firing?

I didn't.

Too be fair here, my poor brass had been abused with hot loads before I knew about measuring the extractor cut area. I know the extractor cut in some of the brass is over expanded because some didn't slide smoothly into the shell holder. I tossed them but there is still an errant piece that shows up once in a while.

Lol, in a way I wish my brass would die off. My barrel keeps going on and on too. Darn thing is still touching splats at 500Y. I'd thought this barrel would have been done last year, nope.

I'm just going to have to pull this barrel off, set it and the brass aside, and go onto another chambering, or else it seems like I'll die of old age before the brass or barrel goes, lol.

 

[Gilbert Garza]

Think everyone read this

http://forum.accurateshooter.com/threads/bra-die-question.3960214/

But it makes sense that minimal amount of sizing would allow for limited growth in the chamber hence providing less pressure on bolt lift.

 

[mikeeg02]

What I found interesting was, say a 30g varget load ran at 2800 during fire forming, then I put 30g in a fully formed case, same exact speed at 2800. Interesting because there is always the thought out there that forming your brass will rob a little pressure and velocity, even in non improved calibers. The opposite of what I saw running that test. So, based on that, I'm not sure any tiny variance in case dimension is going to yield any quantifying result in change of perceived pressure/bolt lift issue. Just a thought.

I think you found a coincidence there. If you were to do the same and form a dasher case, and load the same 30 grains, youd find it was actually slower than 2800. The difference between the case that's forming and the formed br-improved case is the volume.

The virgin piece uses some of the energy to reshape the case.

The formed piece now has a larger combustion chamber than the previous. So the same amount of powder yields less pressure. But since some of the previous pressure went into forming the case, it slowed down the bullet.

For example, a formed br-improved case takes about 31.6-31.7 grains of my lot of varget to make 2980 fps.

My dasher barrel with the same lot and even same jug of varget took 32.7 grains to go 2940 with the same bullets. Only difference is size of the combustion chamber.

As far as sizing goes, I've started to bump mine back until the bolt closes smoothly. If they are any more snug than that, I find that I'll get some slight bolt lift with my moderate charges. But size them so that the bolt closes smoothly, and the bolt opens just as smoothly as it closed with the same load. The whidden click adjustable dies make that very easy to change.