Amazing what effect .001" has on chamber pressure

[Desert Fox]

Ever since I fired my first shot from my 6.5-47 Lapua Target rifle, I'd began to noticed an erratic pressure spike that came and went. I can be shooting two of the reloads with exact same charge of powder and one brass will show ejector marking whereas another would show none. I also noticed large variation on my SD and ES. That leaves me scratching my head. So began another investigative search for the likely culprit.

I began by measuring the neck diameter of the once fired brass. I found that the average reading was around .290. The fired brass brass is tight enough that it will hold the bullet snugly.

A typical brass will have a spring back of around around .001". That tells me that my chamber neck size is probably .291" (this was later confirmed by my smith during my conversation with him). The next thing I did was to measure the brass neck wall thickness. The measurement I'm getting from my Sinclair Neck wall thickness gage varies between .0125" to .0135". I also measured several 130 grain Berger VLD's and they're consistently measured .2645". The average neck diameter measurement of the loaded round is .292". That made my loaded round .001" larger than my rifle chamber neck diameter.

Removing high spot on the brass neck wall got me to my desired wall thickness of .0125". My reloaded round now measured an average .2895 to .290". My last trip to the range did confirm that problem had been solved.

 

[DrMike]

Fascinating demonstration of sound physics. Thanks for posting, Rommel. I appreciate your methodical approach to problem resolution.

 

[SJB358]

DF, that is great intelligence right there buddy. Way to track it and eliminate the problem. I gotta investigate some of your procedures and try to refine my loading a little more. Thank you for sharing. Always a pleasure to learn from you.

What is your speed now? I imagine you are getting what you want out of the bullets for velocity now that you have figured out the necks.

 

[usmc 89]

did you have any tell tale resistance on the bolt when you chambered those with the thicker necks? did you turn the necks down to a uniform o.d.?

 

[joelkdouglas]

What do you consider the ideal clearance? Does it vary by caliber?

 

[257 Ackley]

Df -
Thanks for the info...you will help make us all better. I just bought a neck turner to uniform all my brass for calibers that might have tighter tolerances on the chambers. Talk about tedious work :roll: ... I now have 500 rounds of Lapua brass for my 6BR to check.

 

[tjen]

I never thought target shooters were using preasures at very tip top of the scale. Tight tolerance all around for sure. I lightly read the post were you actual getting a pressed fit on the bullet/neck/chamber when you closed the bolt?

I only load for hunting a fun target shooting so I do not neck turn or have special cut chambers. But the first thing I check is case length on all cases new or used.

 

[bullet]

Yep, desert fox has me beat for dotting I's and crossing T's, hard to believe but here is the proof.

 

[IdahoCTD]

I've actually seen pressure spikes running clearances around .001 to .0015 too so I'd imagine .001 over chamber size would be pretty crazy. I'm surprised you could chamber the rounds as well. With my .30-8mm I run .0025 to .003 and it still springs back enough to hold bullets fine for several reloads. I just clean the necks with a .337 neck. I've done it without cleaning them too but it's a hunting gun so I'd rather have a little bit more clearance.

 

[ScreaminEagle]

Very nice Df. That 6.5-47 looks like a sweet round! Is it built on a short action or long to seat the bullet way out?

 

[Desert Fox]

What do you consider the ideal clearance? Does it vary by caliber?

For me the ideal clearance for a hunting cartridge would be no more than .004" over your chamber neck diameter. However, factory chambered rifle usually run twice that much.

Most Bench and Target rifle have tight chamber, usually smaller than SAAMI spec. This is by design so as to compensate for the variation in brass wall thickness. Brass neck wall thickness varies from brand to brand and even lot to lot. Commercial brass thickness on average range from .012" to .015". I've seen military brass that measured as high as .016". If your rifle has tight chamber, you almost always have to turn the neck wall.

 

[Desert Fox]

did you have any tell tale resistance on the bolt when you chambered those with the thicker necks? did you turn the necks down to a uniform o.d.?

The round will chamber alright. There must have been a slight resistance when cycling the bolt but not enough for me to notice.

The thinnest portion of the neck wall I measured averaged .0125", and the thickest at .0135". I turned the neck just enough to removed the high spot. So the neck wall were all uniformed to .0125".

 

[Desert Fox]

I never thought target shooters were using preasures at very tip top of the scale. Tight tolerance all around for sure. I lightly read the post were you actual getting a pressed fit on the bullet/neck/chamber when you closed the bolt?

I only load for hunting a fun target shooting so I do not neck turn or have special cut chambers. But the first thing I check is case length on all cases new or used.

tjen, even a hunting rifle can benefit from neck turning. I don't necessarily recommend it but it won't hurt, if you decide to do it either.

 

[Desert Fox]

Very nice Df. That 6.5-47 looks like a sweet round! Is it built on a short action or long to seat the bullet way out?

It's built on a short M700 action. The cartridge has a maximum OAL of 2.810". It will fit in the AICS 308 mag with room to spare.

 

[Desert Fox]

What is your speed now? I imagine you are getting what you want out of the bullets for velocity now that you have figured out the necks.

I'm going to the range and chronograph this thing this Saturday. I assume the 41.7 grain will be over 2900 fps since I'm a grain over Franks load that runs 2875 fps. The last time I chronograph the 41.2 grain that's in the picture, the velocity reading I'm getting was around 2868 but showing pressure signs.

 

[bullet]

You know, Frank got lucky when he found you as a friend, but I suspect that you have a lot of fun doing what you do. I appreciated you help on the VLD I wrote you about and I think you got this load really working well with your rifle. Good stuff Desert Fox.

 

[onesonek]

I have been neck turn uniforming the necks on all mine for quite some time now. In part because of tight custom chambers on most. But to also add to what DF said on the benefit to production chamber of factory rifle's.
With their typically roomy chamber, a uniform neck will have even tension allowing for even release. Otherwise, the bullet can enter the lands canted. That single practice does more to aid accuracy than any other in my mind. Accurate powder measuring and consistant seating depth, are close in importance, but I see more effect with neck uniformity.
I also ream the necks, after once fired. How often after, depends on the case design.

 

[bullet]

I turn my necks for my 7mm-08 when I use Rem and Win brass but when I use New Nosler Custom I do not need to turn the necks and my groups on a consistent bases are proof that I do not need to with Nosler brass.

 

[woods]

I have been neck turn uniforming the necks on all mine for quite some time now. In part because of tight custom chambers on most. But to also add to what DF said on the benefit to production chamber of factory rifle's.
With their typically roomy chamber, a uniform neck will have even tension allowing for even release. Otherwise, the bullet can enter the lands canted. That single practice does more to aid accuracy than any other in my mind. Accurate powder measuring and consistant seating depth, are close in importance, but I see more effect with neck uniformity.
I also ream the necks, after once fired. How often after, depends on the case design.

Hey onesonek, what is your method for reaming? And do you do anything to remove all the rough cuts and grooves on the inside of the neck after reaming?

 

[Desert Fox]

Just got my Wilson neck die. Running the fired brass into the Forster full length die induce more run-out than I can tolerate. Although it doesn't affect the rifle accuracy at 100 meters, I'm not so sure what will it do at extreme ranges however.

The runout is very minimal now.

http://www.youtube.com/watch?v=uJrw6o7b ... ture=g-upl