Suddenly Tight Extraction

[CharlieNC]

The 308 has a moderate load of 41.7IMR4064/168SMK, extracted fine last week at 90 degrees. Nothing different (ie no cleaning , same box of loads, etc) and now at a cooler 75 degrees I have to knock the bolt to get the fired brass out. Using the headspace comparator, the brass is stretching from 1.624" when sized to 1.628-1.630" after firing (headspace is set at 1.628"). What is strange is that brass fired last week and 1-2 months ago without sticking problems (have not touched, and it is also 1.628"-1.630") now exhibits the same difficulty. I would swear the headspace has shrunk, but I cannnot measure any evidence of that. Cleaned the chamber with a brush and that does not help. For the first time I see "pock marks" on the bolt face, maybe a burn mark; I will pull the bolt head and polish these blemishes since I cannot find anything else different. Any other ideas?

Finally got this back to the range and after cleaning the throat well (maybe carbon ring) and tightening the headspace back .004" to .003" clearance the same loads shot without and signs of pressure again. Looks like the longer headspace was the culprit afterall.

[eddiesindian]

How about sending a factory round thru it and see what gives. Im almost certain that its not your load, but trying factory loads with the same outcome will defently point you in the chamber/headspace direction.

[CharlieNC]

Been shooting this 3 years, never a factory round. I could see warmer temps are giving high pressure, but recently fired rounds are suddenly tight. Conversely there is ample headspace too. These two facts are too contradictory for me to reconcile.

[barrel-nut]

I have a feeling you've been only neck sizing your brass...? If so, full length resizing is in order. Sounds like you have kinda hot loads and the brass is swelling just forward of the web area, and at the primer pocket. Just my guess. But mine's done that too after several firings without full length sizing, which I do after every firing now.

[BillPa]

I have to knock the bolt to get the fired brass out.

Knocking it how, unlocking the bolt or pulling it back?

Bill

[CharlieNC]

Nut the brass is FL sized LC, and new FC. Plenty of headspace before firing. Bill I can unlock the bolt with considerable torque, then need to knock it back.

[Texas Solo]

I also use FL SB sized LC brass and 4064 with 168's. 41.7gr is the exact point where I begin to get a sticky bolt on my #12BVSS.
I worked backward to find a low node, which was at 38.7. It's fairly accurate, but slow at 2650fps.
I've questioned my use of LC brass many times, but can't afford the good stuff.
Can't figure why your stuff has suddenly changed, but your current situation matches mine exactly.

[earl39]

You are at the bottom of the Hodgdon loading range (41.5 being the starting load) so you could have a light load high pressure deal going on. With the added heat the load preformed better and now you are getting more of an explosion instead of a burn. Speaking just for myself I would try a single round loaded 1/2 grain heavier.

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

[Texas Solo]

You are at the bottom of the Hodgdon loading range (41.5 being the starting load) so you could have a light load high pressure deal going on. With the added heat the load preformed better and now you are getting more of an explosion instead of a burn. Speaking just for myself I would try a single round loaded 1/2 grain heavier.

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

You're discounting the use of LC brass. It can, in some calibers, make a huge difference. 41.5 may be the start load for commercial brass, but I get a sticky bolt at 41.7.
The Hodgdon online manual also shows an FPS of 2518 @ 41.5. I get 2650 @ 38.7.

[CharlieNC]

According to Quick Load with the lower capacity LC brass it is 99.7% full with moderate pressure. I'm running 2650fps too in a 20" barrel. Strange that fired brass that was not sticking last week is today. Bullet is <.005 off the jamb which might make this more pressure sensitive?

[seanhagerty]

Are you trimming the brass to length? If not, you may be crimping the bullet in the throat before pressures get high enough to push it out.

[CharlieNC]

Yes, all brass dimensions verified. The only other thing I have also suddenly noticed is the pits / burn mark on the bolt face. Even with a moderate load I can accept warmer temps have led to excess pressure, but why unprepped-fired brass that extracted ok now does not has me stumped. Maybe warmer temp expansion pushed it over the edge too.

[thermaler]

Well, this is just a guess also--but I think it's unlikely the temperature alone is causing your problems--your pitting on the bolt face is coming from a spacing issue of some sort would be my guess (which can also account for pressure/velocity spikes). I'd ask what do the cartridges look like when you dry-cycle them--do they jam without firing? Might want to run go/nogo gauges too for a reality check.

PS--I've had cases jam that were well short of the over-all headspace in a chamber.

[CharlieNC]

Therm the cases run fine without firing. I rechecked the headspace and it has "grown" from original .003" to currently .007"; do you think this is enough to cause the pressure issue? Thanks for the insight.

[thermaler]

Therm the cases run fine without firing. I rechecked the headspace and it has "grown" from original .003" to currently .007"; do you think this is enough to cause the pressure issue? Thanks for the insight.Could be (depending on what you started out with)--but it could also be due to the cartridge and type of bullet at the same time and that might account for "sometimes does, sometimes doesn't". I'm no expert and am shooting in the dark here--I would start out by firing a solid "middle of the road SAAMI spec" factory round (it won't destroy your bore) and carefully check for signs of pressure/blowback on your bolt face--if there is then I would assume something is going on either with your headspace and/or the bolt is for some reason is not properly closing.

[eddiesindian]

Roger that. Id fire off some factory ammo. If the concern persist?.....then you can rule out your reloads and take it from there.
Bolt/chamber/headspace....

[mtHunter82]

You are at the bottom of the Hodgdon loading range (41.5 being the starting load) so you could have a light load high pressure deal going on. With the added heat the load preformed better and now you are getting more of an explosion instead of a burn. Speaking just for myself I would try a single round loaded 1/2 grain heavier.

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

But with LC brass you have to reduce loads by 10%. I made this mistake ONCE...my brass came out in two pieces, and the bullet soldered the jacket in the barrel, when I started with a medium load (I know...I was young and dumb!).

[earl39]

But with LC brass you have to reduce loads by 10%. I made this mistake ONCE...my brass came out in two pieces, and the bullet soldered the jacket in the barrel, when I started with a medium load (I know...I was young and dumb!).

Everyone keeps repeating this but my LC, rem, and win brass in 308 show less than 1/2 grain difference between the three. That is capacity not weight. I know different lots are different but i am just saying. OP might need to check the volume capacity of his brass to see how far off it is compared to commercial.

[mtHunter82]

Everyone keeps repeating this but my LC, rem, and win brass in 308 show less than 1/2 grain difference between the three. I know different lots are different but i am just saying.

That's what I'd thought too. Some of my LC is even lighter than my Lapua brass...but it creates higher pressures. I think it's the base thickness for the machine guns (there's more pressure on the back of the case from the earlier extraction process).

[jonbearman]

Do you crimp you rounds by chance? It could be a carbon ring has developed from not cleaning it out properly with Iosso or JB Bore paste. If the carbon is present your accuracy will fall off and high pressure could and will be the end result.