backed out primers?

[rayg]

Okay I know that sometimes underpowered loads can cause a primer to back out a bit after firing. However this situation seems different.
I had increased the powder charge from 18 grs of I-4227 powder to 20 grs to see if that would improve the grouping of my 7.7. Jap beagled bullets because the bullets were scattering all over the target. The extra power worked and the groups tightened considerably.
However, the primers had backed out a bit this time where this had not occured with the lower charges. There was no sign of excess pressure.
Some of the previous lower charges even showed black soot on the cases showing the cases didn't fully expand to the chamber but the primers didn't back out.
Any ideas why they are backing out with the heaver charge when they didn't with the lighter charges? Ray

[missionary5155]

My old Lymna book shows a starting load of 28 grains IMR4227... so you are way below.
You Possible have an eccesive headspace situation... Possible you are RESIZING your cases TO MUCH and resetting the shoulder FARTHER back than your particular rifle needs.
I would FIRE FORM 3 cases and compare them to one of your "backed out primer cases"... Then you will know at least if your chamber is "LONG" as compared to how you are resizing cases. Resizing dies are somewhat generic... the will resize to fit any standard chamber.
Personally on my bolt guns I just kneck size and dedicate that brass to that rifle.

[38-55]

RAyg,
I've had the same thing happen with 7.65 arg Mauser.. It's the result of a light load. Case is developing enough pressure to stick to the chamber wals but not enough to set back against the bolt face so the primer 'backs' out. While this probably won't hurt anything you will eventually develop headspace issues as every time you fire it the shoulder will end up pushed back.. As to not backing out with the lighter charges... your extractor was probably holding them in place and your shoulder probably was longer..( see your already seeing the results of what I just said ) Load a mild hot load and that will blow the shoulder out where it should be and probably give you better accuracy to boot..
Hope that helps
Calvin

[rayg]

Oops didn't see your post Calvin until after I posted this.

I understand all those factors and agree with your assestments however why didn't the primers back out with the lower charges? I was using the same cases from that rifle that I had just fired with the lower charge. The cases had only been necksized? I'm sure there's an explaination but it just seems strange the primers didn't back out with the lower charges.
Also I tried COW filler in one of the loads with the 18 grs 4227 and it actually had flattened the primers with that charge showing higher pressurers.
I thought that 28 grs was way too high and not neccessary for the 7.7. cast bullet. If I remember the high end was even higher at 30 grs. I had tried 28 grs once with unbeagled bullets but the accuracy of the bullet was no better then the 18-20 grs I had used in that load. Also there was no backed out primers with those loads except for soot on the 18 gr ones. Ray

[jonk]

I'd suggest that the lighter charge might not have been enough to seal the case to the chamber, as further evidenced by the soot. So the case was driven back flush to the bolt face. No primer backout. The higher charge was enough to seal or obdurate, meaning there was backout, but not enough force to thrust it back after initial combustion and reseat the primer.

[Will]

jonk is right. This happens with a real light load. Nothing to worry about unless you get so light you get something stuck in the tube.

[Pepe Ray]

If you intend to fire very low pressure loads as a general rule, you may be advised to dedicate a batch of brass to that end, by enlarging the primer flash holes to eliminate the primer setback in the "mousefart loads".
Just be careful to keep them marked or segregated to avoid using them in HP loads.
Pepe Ray

[missionary5155]

If the cartridge shoulder is seating properly to the chamber where will there be any room for a primer to back out ? Bottleneck cartriges are supposed to headspace with the shoulder in contact with the corresponding chamber shoulder. Case head supported by the bolt. Granted there is some clearance.. but not enough for primers to back out more than about .005 . When you close the bolt can you feel the bolt pushing against the case ? Or is bolt closing the same with or without a case involved. If your cases are "short" it will not matter if you are just neck sizing as the case shoulder still is not far enough from the bolt to touch the chamber shoulder.
Your primers were not backing out with the lighter charge because primers grip the case (if all is correct) and the load did not develope enough pressure to begin to push it out noticibly.
It is the bolt that holds the case in the chamber. Case walls must expand to SEAL the gases towards the muzzle.. and to some extent GRIP the chamber.. But the bolt must hold the case so the case shoulder contacts the chamber shoulder or soon you will be having case head seperations as the cases keep streching to fill the gap.
Do you have a loading manual that you are looking at for this caliber ?

[Larry Gibson]

Your cases have developed "excesive headspace". This is caused by light loads as the force of the primer explosian drives the case into the chamber and gradually sets the shoulder back. In your 7.7 Jap the shoulder will only set back until the extractor holds the case back and prevents any further setback. The primers weren't backed out with the light loads becase there wasn't enough pressure for the case to grip the chamber walls. What pressure there was pushed the case back and reseated the primers. What you have now with the higher charge is enough pressure for the case to grip the chamber walls so that the case is not pushed back. There is still not enough pressure to push the case head back which would then stretch the case at the web. Thus you have the backed out primers.

Solution; dedicate some of those cases to strictly light cast bullet loads. First thing is to re-fireform the cases. Load a top end load of 4227 with a heavy cast bullet (cull bullets are fine for this) seated so the driving band is well engraved by the leade and you have to force the bolt handle down. Bullets seated backwards so the GC engraves the rifling also work very well. This will hold the cartridge against the bolt face and when fired the shoulder will blow foreward and fire form. If the cartridge is not held firmly against the bolt face the case will stretch at the web and they may get incipient head seperation which you don't want. Once the cases are fire formed deprime the cases and drill out the flashole with a #28 drill bit (that's the largest that is practical for LR primers). A 30 or 31 work fine also. The drilled out flash holes will keep the primer explosion from setting the shoulders back again. Some wory that this may be dangerous but I have thoroughly tested cases with drilled out flash holes and you can load these cases with medium and slow burning powders to upwards of 45,000 psi with cast bullet. I do that with my cases and do not have any problems with the flash holes having been drilled out. That will put you up into the 2400+ fps range with 180+ gr cast bullets which is probably more than your Jap will shoot accurately anyways. Those case are then good for all cast bullet loads, light and heavy.

Larry Gibson

[rayg]

Thanks everone your answers cleared up my question, Ray

[runfiverun]

your primer pockets might be getting loose also.
my 7.7 size die sizes the necks down to 308 specs then back up again.
so i just use my 308 neck die then size up again to 312 for 313 boolits.
the firing pin blow could be causing your problems.
load a regular load shoot and see what your next cast shooting does.