LP Primer In A Rifle Case

[RVTECH]

Just for my own education in case I needed to substitute I seated a LP primer in a 30-30 case and of course being .010 shorter it seated below the rim of the opening. I chambered it and it fired ok with plenty of FP depression but the interesting thing was the primer backed out to where it was slightly above the bottom of the case. I cannot think of any logical reason why the primer backed out and if anybody knows I would appreciate it. Understand now this was a primer only detonation but regardless just because it is slightly shorter why did it back out? Personally the only thing I have considered is when using standard LR primers the breech face keeps it flush but that should happen regardless because the breech is tight against the base anyway. Any help would be appreciated.

Read More

 

[Greenbug]

I think you answered your own question. The primer backed out because it was not supported against the breech face when fired. Generally pistol primers have a softer cup material and that I suspect is why the primer backed out "slightly obove the bottom of the case". Was the base of the primer deformed or was it still rounded like normal? The softer cup material on the large pistol primer used in a large rifle primer pocket could "flow" out of the pocket because it is softer material than is used in a large rifle primer.

 

[RVTECH]

The base of the primer is still rounded like normal and and the level of 'back out' is barely enough to catch a fingernail on. I suspect even with the lack of complete breech face support, theoretically the back out should not be any worse when firing a loaded round because the breech face support is, in effect still there and does not change. Does that 'hold water'? Thanks for the help.

 

[sneakboxer]

Did you just pop the primer or did you fire a round down range? If you just popped a primmer the gasses from the primer will move the primer aft to the bolt face as it is easer to move the primer than get all the gasses through the flash hole. With a normal round the pressure is rise to a point where the case will stretch to the bolt face and reseat the primmer flush.
I learned this shooting rubber bullets in a 38 propelled by primer only. I needed to open up the flash holes to keep the primers from locking up the gun. With a wide open flash hole (just enough meat left to support the cup of the primmer) they did not move.
PS opening up the flash hole will raise pressures so its not recommended unless you are shooting blanks, wax, or rubber bullets.
I think the softer cup of the pistol primer is the biggest issue with their use in a rifle cartridge with upwards of 60k of pressure.

 

[RVTECH]

Nope, just popped a primer and this makes sense as I remember shooting the Speer plastic bullets out of a .357 with just the primer only and the same thing happening. I think I'll try a loaded round with the LP primer and I believe it will wind up staying seated due to the case stretch as you suggested.

 

[AMProducts]

This is a common "condition" for low pressure ammo... full power loads, the casing will be pushed backwards against the breech face, which pushes the primer back in.

You can safely substitute LP primers for LR primers provided you're not running high pressure. One of my pet loads for my .308 is 8grs of clays under a 150gr cast bullet. I typically get primer setback with this load as it's maybe running 12KPSI, I typically only need to neck size the case afterwards, as it barely affects case geometry.

The two things to look out for when you're substituting primers: cup thickness/hardness is a major problem, however at moderate (read pistol) pressures it shouldn't be an issue. The other one is piercing of the primer cup, this can be an issue as most rifle firing pins are meant to deal with rifle primers, and handily pierce the thinner/weaker cups of pistol ammunition especially at elevated pressures.

 

[Norwester]

Sneakboxer and amproducts nailed this one. Upon firing the primer backs out of almost any case until stopped by bolt face, case slams back against bolt face and reseats primer before case can even expand. I believe Phil Sharpes bible was earliest mention of this process I've read. Hope we all dont have to go this route someday.
Happy Loading and Shooting all.

 

[Norwester]

I might add that a LP Mag primer "may" be thicker, and different brands may be "tougher" but have no knowledge that's true, anybody?

 

[AMProducts]

That's a pretty tough call to make, as there are really no standards for cup thickness, charge, and a number of other important factors, the only SAAMI standards for primers are dimensional.

There are two major things that come into play when you're playing musical primers... first is cup temper (this is the grain structure of the material and affects hardness and is actually a more important factor than thickness when it comes to mechanical strenth), the second is the amount of explosive material that is in each cup. This is a very important factor when it comes to pistol vs rifle primers, in that pistol primers do not have the same ignition characteristics of rifle primers. So a pistol primer might not adequately ignite all the material in the case. This can result in some very irregular ignition characteristics.

 

[rgold1963]

Here's an interesting set of images someone took showing different primers igniting: Primer Flash Test Photos
Quite a difference between brands. This does not show cup thickness or strength but there are some other links here that do:

Primer cup hardness comparison - Primers And Pressure

Primer performance comparison:
Small primer - <broken link removed>
Large primer - <broken link removed>

 

[ma96782]

RVTECH,

For liability sake.....just use what your manual says to use.

BUT............

Since it seems like you've been doing some experimenting w/ primers......

And BTW, I'm not saying to do this with rifle ammo. There is RISK involved. So, take appropriate precautions when conducting any experiment.

Here is the experiment.......

Try drilling out the flash path/hole a little larger. There will be less back pressure to push the primer backwards. Wax pistol bullet shooters use that trick to prevent the primers from backing out and jamming up their revolvers. Just remember to mark the brass so that a "mistake" doesn't happen with full power real bullet loads.

That all being said, WHY are you loading pistol primers in your rifle cases? If you lack the proper large rifle primers, why not just trade your primers for the correct ones?

Aloha, Mark