Where in the HE!! are these pressue signs coming from !!!

8541 said:
I have been chasing my tail with this rifle for 125+ rounds....it does the same thing whether it is H50 BMG, or US 869. The actual powder charge does not seem to matter. I have had 105 grains of each powder cause the same issue, and I have had 108 grs shoot fine.
Click to expand...

8541 said:
The issue "seems" to happen in a string. Again, it seems that things are going to go well, or not; 3000 fps...... or 3080 to almost 3200, stuck bolt etc. I have already pulled the rest of the loaded rounds and the the powder charges are all right on....fwiw.
Click to expand...

John,

I have read this post a few times and after rereading a few things stand out. First is your powder charge, not that it is excessive but it is a lot of powder and BTU’s. Second it happens when shooting a string. I shoot a 6 Grendel in my heavy gun at 600 and 1000 yards in benchrest and typically run my shots. A 10 shot group would take between 15-20 seconds to fire and I shoot less than 1/3 the powder of your case.

Another shooter who did not do so well on the first two days saw my success and tried running his shots the same way. Unfortunately he was shooting a 284 Shehane and his barrel heated up rather quickly. As his 10 shot string went on his problems compounded over the ten shots. He started blowing primers, not just blowing them enlarged primer pockets, sticky bolt and other high pressure case problems. A lot of BTU’s in a short period of time. Now when he shot slowly the first two days he had no pressure problems at all and the temperature was about the same. Once he started running his shots the problems arose. He had a 2” barrel that tapered down to 1-3/4”? not sure of the muzzle diameter but the chamber was 2” in diameter.

You shoot a large volume of powder, only happens when you shoot a string of shots I can’t help but wonder if the problem is the barrel overheating. When you shoot a few rounds or spaced apart you don’t have any pressure problems. Something to think about.

John
 
John,

what exactly would "the barrel heating" have to do with pressure? I can't see a mechanism that would cause a hot barrel to make pressure.

al
 
Al,

Think about it?

His pressure happens in a string, only with the 240 grain SMK and not with the lighter bullets. The 240 SMK’s have a longer bearing surface, create more friction in the bore as the bore heats up creating more pressure behind the bullets and in the chamber. This may lead to higher pressure in the barrel and chamber causing his high pressure.

It seems as if everything has been checked and rechecked to ensure everything is proper and nothing has been found. Lighter bullets, shorter bearing surface no problems shooting a string. Heavier bullets, longer bearing surface shooting a string causes high pressure. If all else is the same, cases, rate of fire why do only the heavy bullets give pressure problems when shooting a string?

It would be interesting to know if the pressure problems all come around the same time in a string. Is it every 5th or 6th shot in a string where he starts having pressure problems? Is it random and sometimes come at 5 shots and others at 20 shots. If the pressure signs come at the same time in firing a string the long bearing surface with a heavy bullet may be the culprit for building high pressure.

I am not saying this is the problem, bringing up what might be an issue and another venue of thoughts to consider.

Al, do you have any suggestions why the heavier bullet with the longer bearing surface is creating pressure issue? Or why there is no problem with the lighter bullets with a shorter bearing surface? All using the same cases.

John
 
JMC said:
Al,

Think about it?

His pressure happens in a string, only with the 240 grain SMK and not with the lighter bullets. The 240 SMK’s have a longer bearing surface, create more friction in the bore as the bore heats up creating more pressure behind the bullets and in the chamber. This may lead to higher pressure in the barrel and chamber causing his high pressure.

It seems as if everything has been checked and rechecked to ensure everything is proper and nothing has been found. Lighter bullets, shorter bearing surface no problems shooting a string. Heavier bullets, longer bearing surface shooting a string causes high pressure. If all else is the same, cases, rate of fire why do only the heavy bullets give pressure problems when shooting a string?

It would be interesting to know if the pressure problems all come around the same time in a string. Is it every 5th or 6th shot in a string where he starts having pressure problems? Is it random and sometimes come at 5 shots and others at 20 shots. If the pressure signs come at the same time in firing a string the long bearing surface with a heavy bullet may be the culprit for building high pressure.

I am not saying this is the problem, bringing up what might be an issue and another venue of thoughts to consider.

Al, do you have any suggestions why the heavier bullet with the longer bearing surface is creating pressure issue? Or why there is no problem with the lighter bullets with a shorter bearing surface? All using the same cases.

John
Click to expand...

I did think about it.
I have thought about it.
That's why I asked the question :)
al
 
The 240 Sierra has a lot of bearing surface and you may be hot. I don't know for sure because i don't shoot that case. When i shot the 240's in competition we ran the bullets through a neck bushing to check diameter. Some of them would fall through and some were snug and some you couldn't push through with all you could push. Some lots were better then others but if the bullet was fat you would get vertical fliers. If you have uneven turned brass or are not turning all the way to the shoulder on some this may be pinching the bullet on some cases; especially if the pressure ring on the bullet is back past or at the neck shoulder junction. If it is a .333 neck in gun the brass should be turned to 11 thous. wall minimum. Matt Kline
 
Last edited by a moderator:
Anomilies with these bullets has always been on my list of suspects. So have NK tension issues, and the carbon ring. I will keep this thread alive and keep giving feed back as I test and test and test.....trying to figure this out.

Thanks for the help gents. Lots of valuable information you guys have suggested I need to investigate further.

John

One last question....what is a good bang for the buck bore scope?
 
John: Hawkeye Borescope I never leave home without it! Sinclair has them for $850 with the hard case. Someone may know were to get it cheaper. But you may also want the 90 degree eyepiece. Still wondering how thick are your necks!

Joe Salt
 
850.00 !! This shooting stuff is as bad as my racing habit !!

Necks are .011 to .0115. .330-.331 loaded rounds
 
In looking back, I would have to agree with Charles' evaluation of my answer. Sometimes I skim too rapidly, and focus in on the problem without proper attention to the important details. Obviously, he is right about the belt.
Boyd
 
Boyd, It's not a bad thing to check. In fireforming cases, all kinds of crazy things can happen. I remember telling Jackie that if he'd fireform his .30 BR a certain way, cases wouldn't shorten up much. Don't think he believed me at first, then he found an even better way, & I don't think they shorten at all.

In this situation, though the belt would stop forward movement of the case, & by the time I replied, the poster said he'd checked for other "lengtheing causes."

For all of us, Boyd's point should be taken, and even if there is a belt, check the fireformed cases for length. Just common sense.
 
Charles: yes that has always been part of my reloading sequence after there all sized, they go through my wilson trimmer whether they need it or not just so I know. Better than picking them up one at a time and measuring. Then there done! I also found out even without the belt you can still have the miseries of expansion using the wrong brass in your rifle. I would also suggest John try some H-4350 in his 30-378, I have had great results in the 308 Baer, I believe it is one of the most consistent powders out there, and people that think you have to fill there case with a slow burning powder are wrong!

Joe Salt
 
Not offering any solutions...just adding that I had this same problem in another life with a 6mm-06. Tom fool thing would operate flawlessly and then without warning it would stick a case hard, blow the primer clean out and wipe all the writing off the headstamp. I don't know what the actual cause was but dropping the powder charge two grains stopped it from happening. Additionally, I soon learned to hit a pork-n-bean can at 75 yards 3 outta 5...just before the crosshair broke (disappeared) in my scope. Was a great day when I traded that thing off....I did miss the noise it made and how it would pull up grass if you shot it kneeling.
 
Last edited:
You must log in or register to reply here.
Back
Top