300WSM extraction problem at 1K match...

[chuckgreen]

At our local 1K match yesterday I started getting a really stiff bolt lift problem about half way though the match.

I was shooting 215 Berger Hybrids over 60.8gr of H4350 lit by 215M primers. A load that usually gives me about 2,850fps with no pressure signs during testing. Brass had been fired once before and the barrel had about 200 rounds on it, 15 since the last cleaning. I use the same light gun for both LG and HG classes so will fire close to 80 rounds through it during a match. Yesterday was the first time I had used this barrel and load in competition. Ambient temperature was about 75 degrees.

The rifle functioned perfectly for all of the 40 rounds used for LG and for about the first 15 rounds of the HG portion of the match. But...during the second 10 round HG string, as the barrel started getting hot, I started getting a stiff bolt lift and by the end of the third HG string I was struggling to extract the fired cases. By this time the barrel was getting very, very hot, of course.

This problem really slowed down my strings and negatively effected my score. Not acceptable.

I'd really appreciate any thoughts on how I might eliminate this problem in the future. Thanks...

Chuck

 

[rsmithsr50]

do you have a bbl cooler ?
seems that something might be a bit temp sensitive size wise, back of the chamber

 

[Laurie]

I'd run your loads over the chronograph again. Some makes of stainless barrel appear to both 'speed up' and produce higher pressures around the 150 round mark plus or minus 50 shots.

Some people claim this is a shooter's myth, but it seems to happen with some examples. I know of two cases in the UK with very experienced and capable bench shooters and handloaders both using low round count Benchmark 6mm barrels. I saw it myself recently with a GB Barrels (UK make of cut-rifled barrel) in 300 RSAUM where MVs rose by nearly 50 fps from the immediate post run-in figure. As this was a mild load, there were no pressure issues, but it apparently took my initial load out of the tune-node, so after poor performance in an F-Class match I returned to the short-distance range to see if anything had changed.

 

[ben_g]

I too have seen MV increase between approx 150-200shots on a few barrels, for example in my last 6mm dasher I ultimately needed to drop 1.5gr from the initial load to maintain desired MV.

The other thought with using H4350 is possibly carbon fouling. But this is less likely given how new your barrel is.
I use H4350 in my 284’s and SAUM and I find it fouls heavily with carbon and those barrels need a paste every 200 round or so (on top of normal cleaning with bronze brush after days shooting) to keep it at bay. If that fouling is left to go too far accuracy suffers and pressures rise.

I use RE23 in my 300WSM and find it fouls very little compared to the the other barrels with H4350. Paste is needed less frequently.

I think Laurie is probably on to it however. I would thoroughly clean the barrel then chronograph the load and reduce accordingly if it has increased in velocity.

 

[chuckgreen]

Thanks to all for your thoughts and suggestions. I'll give the things mentioned a try.

 

[gunnermhr]

I'd really like to hear Matt's input on this. I have experienced something close to this and believe it's a sizing issue. I'm pretty sure Matt uses a custom seater which eliminates this issue. Matt?

 

[mikeeg02]

Did the stiff bolt lift start at initial opening of the bolt? (full travel of the bolt handle) Or at just at the top where its considered a "clicker" ?

What did the primers look like? Any ejector marks?

Were the first ~200 or so rounds all fire forming?

Is this the first round of brass that had been fired once and then sized?

I'm pretty sure Matt uses a custom seater which eliminates this issue. Matt?

I think you meant to say sizer.

 

[BIGMAC]

Will be interested in Matt reply. Do you think he is using a custom FL sizer or seater or both.

 

[46and2]

Another question, you said this brass was fired once before, that means that firing was virgin brass.
IME the load usually requires some tweeking on fire formed brass.
One train of thought is, On the first firing the brass expansion acts like a 'cushion' absorbing some pressure. That 'cushion' is gone after that and is compounded if the brass is neck sized only. Propertly full length sizing by bumping the shoulders a couple of thousandths give the cushion back.

 

[gunnermhr]

Will be interested in Matt reply. Do you think he is using a custom FL sizer or seater or both.

Both, I meant sizer, I think that eliminates most issues. The custom seater helps with runout only

 

[chuckgreen]

To answer a few of the questions that have come up on this thread....

On a barrel cooler... I have one on order. A Chamber Chiller that is supposed to really move some air down the barrel. Hope it will help.

On the bolt lift... It was fine for the first 55 rounds and then started getting stiffer and stiffer for the last 25. Initially the stiffness was just at the very top but then increased to include all of the bolt lift movement.

There were ejector marks on all the cases...more pronounced towards the end and the primers started to flatten more towards the end of the match. However, I'm using soft Norma brass and seem to get ejector marks with almost any load. I saw no real signs of excess pressure for the first 55 rounds. Then, as the barrel and chamber really started heating up during the HG strings it seemed like pressure started to increase more and more.

While testing I found that I got clear signs of excess pressure at 62gr of 4350 but when I reduced the charge to 60.8 all seemed to be fine. I had not, however, tested that charge in a hot barrel.

100 of the rounds fired previously through the rifle were fire forming only. About 40 of the match rounds used for LG had been fired a second time during testing. All of the cases on the rounds used for the HG strings had been only fired once before to fire form them. All the cases used that day had been full length resized and the shoulders bumped back but not with a sizer custom made for the chamber. I do have a custom Whidden sizing die on order being made based on fired cases I provided. Still a couple of months out.

I hate to mess with the load as the gun seems to like it. Despite the extraction hassles it shot a 98XXX for high match score on one HG target and won total three target HG score. I'm hoping that perhaps just finding an effective way to keep the barrel cool may resolve the issue.

Thanks to all for your interest.

 

[ben_g]

Are you cooking the rounds in the chamber before firing?

If not I doubt a hot barrel/chamber would transfer much heat to the powder to spike pressure.

 

[Ringostar]

How much neck clearance on loaded round to chamber? You could possibly have a carbon ring build up, that is now getting tight on a loaded round.

 

[chuckgreen]

No on cooking rounds in the chamber. I run my strings as fast as possible so a loaded round is only in the chamber a couple of seconds.

I think I have plenty of neck clearance. Chamber has a .334 neck and I'm using a .332 bushing. With relatively few rounds down the barrel and regular cleaning tightness in the neck should not be the issue. Feels more like tightness at the base of the case.

I'm planning to get to the range before the next match and do some experimenting. Shoot some rounds over the Chrono with the barrel relatively cool and then purposely get it hot like at a match, shoot some more rounds with the same load and see if velocities and pressure signs increase enough to duplicate the extraction problem I had Saturday. Then, when I get the barrel cooler I ordered, I'll do something similar and see if the cooler makes a difference. Leaving the load alone if the cooler resolves the problem would be the preferable solution for me.

 

[Boatschool02]

0.334 neck... what does a loaded round measure?

 

[NA Woody]

I bet you need more neck clearance. Can you drop a bullet into a fired case neck? If not I would turn those necks down for more clearance.

 

[mikeeg02]

334 neck means your neck walls max thickness can be .013" (not counting the pressure ring). Now to add a couple thou clearance (.002) they should be closer to .012". Which with a bullet would measure .332". If you're using a 332 bushing, and still getting firm seating pressure, as suggested above your necks are too thick for the chamber.

 

[chuckgreen]

Sorry...I made a typo. Chamber neck is .337. Neck thickness after turning is .0125 and a loaded round measures .335.

 

[Bc'z]

Sorry...I made a typo. Chamber neck is .337. Neck thickness after turning is .0125 and a loaded round measures .335.

That's still only giving you .001 clearance around the neck.
I'm having the same dilemma in my 06' chamber. I'm thinking you need to have .004 neck clearance to chamber giving you .002 clearance all the way around.

 

[chuckgreen]

Well, could be a little more clearance would be better. I had not seen any problem before the incident with the hot barrel. The loads I have been using chamber easily and during the subject match the first 60 rounds I fired cycled and extracted with no problems. Might try some clearance next time.