Case separation questions - .280 A.I.

[Jesse09]

I think this will be a challenge to solve; I am loading for a Single Shot Pistol, a falling block design made by M.O.A. Corporation. I'm using Hornady brass, Nosler 140gn AB, CCI BR-2 primer and 62.0 gns H4831SC. I set headspace by "feel": I adjust the Redding Body Size die until there is a very slight drag felt on the lever as I close the action. My best guess is this gives me .000" of headspace, +/- .001" depending on the individual brass. After, at most, three loads there was the typical thin external ring and incipient base separation (I dragged a hooked wire inside the brass). For what it's worth, the maximum load listed on the Nosler site for this bullet and powder is 62.5 grains. During load development I did test 60.5 and 61.5 grains of the same powder, but the groups didn't come together until I shot 62.0 grains.

I don't think this is either a headspace or "hot" load issue, but can't think what else it could be. I don't want to fault the Hornady brass because I've never had an issue with this brand across the many calibers I've used it. I can't reduce the load because the smaller charges didn't group well. Is there any reason to think that starting over with a different powder, say IMR 4350 (I have it on hand) would eliminate the case separation? The only way I can see that working is if I can reach the required velocity and group size with a charge weight that generates less heat and pressure.

Also, just for my information, what are the possible effects of "negative" headspace?

I'd very much appreciate any thoughts or corrections in my thinking.

 

[Mikecr]

The thinning represents a case reaching back too far to the bolt face. This is something that an Ackley Improved case, in particular, is not prone to. Head spacing should also be easier with this design, even by feel.

What's happening with first fire forming?
Do you have the normal crush fit for this?
Is there a lot of diameter growth and extra sizing for this, to begin, and each sizing cycle?

I do think you'll need to measure things carefully to solve this.

 

[Jesse09]

Thank you for the reply, Mikecr;
1. There is no "bolt face", per se. There is instead a channeled block that contains the firing pin assembly. This slides up to close the chamber behind the brass when the lever is worked.
2. I'm not sure what you mean by "case reaching back too far to the bolt face". As stated, the case is adjusted to a slight drag against the block.
3. I don't fire-form. I buy .280 A.I. brass.
4. There is relatively little sizing required.

 

[Tripodmvr7]

Are you referring to a falling block similar to the Ruger no. 1?

 

[nmbarta]

Measure a fired case using a heads pace gauge, and compare against an unfired case so you know how much your case is growing when its fired. Thats the first piece of information you'll need.

 

[ShtrRdy]

I would suggest cutting a case in half, the long dimension, to have a look at web thickness.

Several years ago I bought some once fired brass from a private range. It was Remington brass in 300 Win Mag. I sized it just enough to fit my chamber. Upon loading it up and firing it I had a few pieces partially separate toward the base. I found that the brass was very thin in the web area. Not just at one point but throughout the web.

 

[243winxb]

I buy .280 A.I. brass.

A comparitor is needed to measure the new brass, head to datum, that you buy. Write it down. Now fire that brass with your maximum load of 62 grs. Measure this fired brass and compare to the new brass. If the stretch is over .010" the brass may be getting damaged on the very first firing.
Then during the next 2 firings, it separates.

Will fired brass fit back into the chamber? If not, pressure is to high for that type of action.

Different lots of H4831 can have a 10% spread from slowest to fastest. This means, if the data was worked up with the slowest lot, and your lot is 10% faster in burn rate, your way over maximum pressure.

 

[243winxb]

Oops

 

[Bob Wright]

Measuring by "feel" in your chamber may be giving you a false sense of security.
Measure new brass or factory cartridges for the base to shoulder datum (comparator and calipers).
Shoot one and measure again. How much growth?
Trim length, a carbon ring, a tight neck clearance all can give you false "feel" with no valid measurements.

 

[Jesse09]

Thank you, all. Yes, this action is similar to a Ruger #1. It seems everybody is in agreement that I need a headspace comparator, so I'll order one. I already have the Loc-N-Load (old Stoney Point) body and caliper attachment, so will try to order the correct bushing, only. The little I just read online says to determine the correct bushing by adding the neck and shoulder diameters of unfired brass and dividing by two. Am I measuring the shoulder just before it tapers down to the neck? Also, since I don't have any new, unfired brass I guess I have to order new brass before using the tool. Is this correct?

 

[Bob Wright]

Only mentioned unfired due to the mentioning of suspect brass you may have. If you can't find any, I have one I can send. PM me.
The comparator datum line and diameter is the same for most of these using the parent 30-06 cartridge. Hornady website may be helpful here on comparators.
Stony Point data should be out there as well.

 

[243winxb]

.375" datum for .280 AL. A 40S&W case can be used for 30-06 See photo at link. https://www.longrangehunting.com/th...ulder-bump-without-rcbs-precision-mic.241959/

 

[Jesse09]

Thank you, Bob, for the offer of brass. I just found a handful of neck-sized only new brass that I had put back because it was a little shorter than the "trim to" length.
243winxb, you've given me some good direction. That's a neat trick with the 40 S&W brass, but I don't have that caliber. If that case will work, then I don't see why I need to be so exact in my measurements; I'll just plan to order the .375 bushing.

I'll share my findings on this thread. I appreciate your all being so helpful.

 

[Bob Wright]

Thank you, Bob, for the offer of brass. I just found a handful of neck-sized only new brass that I had put back because it was a little shorter than the "trim to" length.
243winxb, you've given me some good direction. That's a neat trick with the 40 S&W brass, but I don't have that caliber. If that case will work, then I don't see why I need to be so exact in my measurements; I'll just plan to order the .375 bushing.

I'll share my findings on this thread. I appreciate your all being so helpful.

You should be bumping your shoulder .002-.003 when full length sizing. You need to be accurate on your caliper/comparator within .0005 or less It's more important to be comfortable, a good feel when your measuring to get precise measurements. I've seen guys mash down on a micrometer or caliper like they are using a vise. Dont be that guy, lol. Practice returning to ".000" when calibrating you caliper and take multiple measurements to get the feel on your brass.