Pressure signs with my 30-378 as the barrel warms

[Sgt. Brumbaugh]

I have an issue with my 30-378 that I am having a hard time getting a handle on. The rifle is a custom long range rig

Hall G
34 in Pac-nor 3/3 1.25in bull barrel
.333 neck.

I am shooting 240 SMK at around 3050 fps when everything goes right, but I get weird pressure signs out of nowhere. I have had this happen with powder charges from 103-108 (H50BMG) I went out today and shot 13 rounds and everyone (except one) was faster then the last one. I shot a 5 shot group, let it sit 5 minutes, then a 3 shot group, let it sit 10 minutes and finally another 5 shot group. The velocities ranged from 3038-3106. Anything above 3070 or so and the bolt gets a little sticky and the ejector leaves a nice shiny reminder the pressure is too high. The ES seems very high considering each piece of brass is fully prepped, annealed, necks cleaned, trimmed, etc. The same procedure in my other rifles will yield single digit ES's.

I have fought this issue every time I have taken this rifle to the range. I have reduced the load from 108 down to 103 and beleive it or not it does not seem to make much of a difference. Temps when shooting have been from 70's to high 80's, again this did not seam to matter either. This time to the range I weighed each powder charge on two different scales (103 grs.), measured every neck clearance (I run .002-.0025) and every seating depth (+ or -) .004 total variance. Nothing obvious other than as the rifle got warm the velocities went up until things got unhappy.

I called Hodgens and asked them if they have had a bad batch of H50BMG and they say no.

I remember when I was in the Marine Corps our first generation 50 cal's would do this. The bolt would get sticky as they warmed up, but this was with a pretty crappy rifle and military ammo, none the less it did happen.

My next step is to try some Retumbo or at least another powder.

Any other ideas?

Thanks,
John

 

[Broz]

Re: Pressure signs with my 30-378 as the barrel warms

Retumbo will do it too. Maybe worse. I would try this.

Try H-1000 and let cool longer periods. 5 or 10 min. after a 5 shot string is not enough for this caliber. I would also try to limit strings to 3 shots max with a cool time of 20 to 30 minutes. Once you get the chamber and tube good and hot it takes a long time to cool at 70 to 80*. Also if you are chambering a round and waiting quite a while before taking the shot you are cooking the round with a warm chamber rounds need to be fired as soon as you can get solid on target.

Jeff

 

[Sgt. Brumbaugh]

Re: Pressure signs with my 30-378 as the barrel warms

Thanks Jeff. I will give the H1000 a shot along with longer cooling periods.

 

[nuclabuyer]

Re: Pressure signs with my 30-378 as the barrel warms

I also had pressure problems on my 30-378 pac-nor barrel. Mine is a Sako M995 w/ a 32" barrel. I called them and I had to send it back to them and they added a touch more free bore and it has been good since. Mine however was over pressure with the lightest loads.