Question about High pressure factors.

[Iwillylike2shoot]

I had a macgowan barrel blank spun up for my 700. It's a 6.5 creedmoor. I am having issues with high pressure. My first few shots were with factory ammo and there were all way over pressure (very difficult bolt lift, hard extraction, plunger flow) then I loaded some middle of the road 130 and 140 gr loads and found they were to hot as well. Different bullet weight/ design seem to produce wildly different results on pressure. My loads now are down at the bottom of the spectrum according to all the published data I have found. Right now I'm thinking it could be a tight bore or a tight spot in the bore. What else could it be?

I'm about ready to just send it in and have it re barreled.

 

[supercorndogs]

Too much head space, bullet into the lands.

 

[918v]

+ not enough neck clearance.

 

[spife7980]

Some measurements would be really helpful in trying to diagnose where they are out of place.

 

[Iwillylike2shoot]

Some measurements would be really helpful in trying to diagnose where they are out of place.

Measurement from fired brass?

 

[Iwillylike2shoot]

+ not enough neck clearance.

What is the difference between neck clearance and distance to the lands?

 

[spife7980]

Measurement from fired brass?

From virgin and from fired and then see where those differ from what the chamber specs should be.

What is the difference between neck clearance and distance to the lands?

One is a diameter of the neck and one is how far the bullets are from the lands.

 

[Iwillylike2shoot]

From virgin and from fired and then see where those differ from what the chamber specs should be.


One is a diameter of the neck and one is how far the bullets are from the lands.

I'll get some measurements

 

[Iwillylike2shoot]

It's deer season here in NE so I will get back to this on Monday with some measurements when it is legal to be out and about shooting without a deer tag.

 

[supercorndogs]

Probably the OD of the neck on a fired and loaded round would be first.

 

[Iwillylike2shoot]

Slow it down.

I mean your patience. I have 5 McGowen barrels. They all exhibit high pressure during break in and they all speed up considerably. On my 6.5 SST barrel I'm down 2grs of powder after 160rds to achieve the same velocity and without pressure. With my 22 BR I'm running 1 gr more powder at a faster velocity, without pressure. If I'd tried to run this load when the barrel was new I pierced primers.

Give it 140 to 180 rds

I have more than that. I think I'm pushing 300 now. I'll check my data book when I get home.

 

[Iwillylike2shoot]

From virgin and from fired and then see where those differ from what the chamber specs should be.


One is a diameter of the neck and one is how far the bullets are from the lands.

Should I use some loaded factory ammo I have to get my before and after measurements or should I buy new virgin brass?

 

[Iwillylike2shoot]

Well, in that case I retract my post and advice, ha ha. Might be a slow barrel....

Round count is 371 on the barrel right now. Thanks for the input though it might still have some relevance to what is going on and I appreciate it.

 

[Iwillylike2shoot]

	Length	Neck D	Web
Loaded Factory GMM	1.535	.292	.464
Fired GMM	1.539	.296	.469

So I found a partially shot box of factory and got some measurements. From my perspective this doesn't look bad but I'm interested to see what you guys think. One other measurement I took was the base to ogive on my hot hand loads and factory ammo. I found that my hot hand loads were 2.195 and the factory is 2.198 so I wonder if my throat is short and I need to seat my bullets back further from the lands like @supercorndogs thought.

 

[spife7980]

• Your case web starts .464 and expands to .469, that sounds just fine. Saami chamber max is .471 so you have room there. Coating a piece of fired brass in sharpie and trying to chamber it will show you if its actually too tight for your real world chamber instead of these prints. If it rubs the marker off all the way around the case then you know thats where you have interference rubbing.

• Your brass neck is .292 and grows to .296. Saami chamber calls for .297 and tapers down to .296 which is what your fired brass is left at. Seems fine to me. You have .004 of clearance between loaded and fired so thats fine.

The case shoulder position is a bit tough to judge off a print when comparing to your comparator numbers. If you have a go gauge I would measure that with your comparator and then see how it compares to your fired brass. Because the comparator inserts can all be slightly off its a relative measurement and not an absolute measurement. In this case the go gauge is known to be 1.630 but it reads 1.624 meaning any measurement you get on your brass is also skewed this much from the real world absolute measurement.

• So your case should be from 1.5438-.007 meaning from 1.5438-1.5367 and your chamber should be from 1.551 to 1.541 when justified to the go gauge. A maximum case can measure 1.5438 and a minimum chamber can measure 1.541 so it is possible for some max-in-spec ammo to not fit in some min-in-spec chambers. You are getting 1.535 which grows to 1.539. Thats not directly comparable to the spec without a go gauge to be certain... but they dont look overly egregious. I would trust what your comparators are telling you in that you have a .004 relative measurement of case growth from virgin to fired.

Now, your bullet ogive measurements. Are you using the same bullet in your reloads that you are in the factory? If not its tough to compare bulletAs position relative to bulletBs. Like needing to measure the go gauge you would need to establish a standard to go off of such as finding your distance to the lands with bulletA and then finding it with bulletB and comparing the difference between the two.

I prefer the wheeler method for finding the lands. Let the materials tell you when they touch themselves instead of your fat monkey fingers trying to gauge it.

 

[Iwillylike2shoot]

@spife7980 that is a some great info! The bullets in the loaded ammo are not the same. I will try to find the lands and see what I get for results.

 

[Iwillylike2shoot]

So I tried those same hand loads yesterday after I seated some 0.015 deeper. I ran 6 through as fast as I could and the first 5 ran fine. The last one was a little sticky but I'm sure the barrel was getting warm at that point. I would say I found my problem. Thanks for all the info though I have learned a lot.