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Big magnums developing pressure signs as barrel warms?

 
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  #1  
Old 10-02-2011, 09:43 PM
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Big magnums developing pressure signs as barrel warms?

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.

Thanks,
John
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  #2  
Old 10-02-2011, 10:18 PM
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Re: Big magnums developing pressure signs as barrel warms?

Here is a pic of todays 100 yard target. The top row is my short barrel .308 pack rifle with 200 NAB's. Too slow of powder (RE17) but good accuracy potential for a 20 in barrel less than 7 lbs rifle.

Not too make excuses for the fliers; but I had a group of guys with their M-14's, etc shooting rapid fire right next to me. It is hard for me to shoot tight groups when I have BANG, BANG,BANG right next to me. I am sure the 30-378 is a 1/4 MOA rifle when I can do my job. It should really shine at long range when I can get this pressure thing worked out.
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Big magnums developing pressure signs as barrel warms?-oct2testing.jpg  
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Old 10-02-2011, 11:31 PM
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Re: Big magnums developing pressure signs as barrel warms?

Quote:
Originally Posted by Marine sniper View Post
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.

Thanks,
John
Not sure , but it sounds like it is getting more and more fouled.

Clean it to bare metal and then shoot it one time and clean. repeat the process until you have shot
6 or 7 times and cleaned each. do this through your chronograph and see if it makes a difference.

Also when you close the bolt on a round shoot it as soon as you can so it doesn't have time to heat up the round. The big 30 will heat up the chamber after only 2 shots.

Just something to try.

J E CUSTOM
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  #4  
Old 10-03-2011, 08:00 AM
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Re: Big magnums developing pressure signs as barrel warms?

What J E said?
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  #5  
Old 10-03-2011, 09:51 AM
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Re: Big magnums developing pressure signs as barrel warms?

I will give it a shot. Thanks.
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  #6  
Old 10-03-2011, 12:20 PM
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Re: Big magnums developing pressure signs as barrel warms?

Quote:
Originally Posted by Marine sniper View Post
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.

Thanks,
John
those big mag cases don't like to be down loaded much, and really like to be loaded close to max. I'd try 111.0 grains of AA8700. Those cases like real dense loads
gary
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  #7  
Old 10-03-2011, 12:49 PM
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Re: Big magnums developing pressure signs as barrel warms?

Gary (from yellow bullet?)

AA8700 is faster than H50BMG. The only slower powders Hodgens lists is US869 and VV 20N29. I am at approx 90-95% density now. I "think" JE might be on to something, the barrel has 85 rounds through it and cleans very easy, but his idea sounds plausible, particularly when I review my date. I will give it as shot and see what happens
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