Shoulder set-back?

[ShooterAZ]

I read in the military rifles section about the occurance of shoulder setback when repeatedly using mild loads with cast boolits in rimless rifle cases. It was explained that primer blast shoves the shoulder back. Enlarging the flash hole was recommended.

My questions:

1) How many firings before someone needs to be concerned about this?

2) Is there a particular velocity/pressure to adhere to to prevent this?

3) Do any of you guys enlarge the flash holes?

4) My current load in both 30-06 and 308 is 16 grains of 2400 powder with a 155 grain boolit. This load seems mild to me...too mild? Would I need to be concerned with setback with this load?

Thanks in advance...

Shooter

[stubshaft]

Not to say it doesn't happen but, Ive never had any problems with it. The premise as I see it is that the primer upon being hit by the firing pin. Will back out slightly and push the case forward into the chamber setting the shoulder back. I have enlarged the flash holes in some cases but only to use the primer alone as the driving force. Like gluits and to shoot pellets.

[trk]

Enlarging the primer hole will do two things: increase velocity slightly and increase the pressure greatly. Use them with the regular load/bullet and the pressures go dangerously high. Phillip Sharp documents this in his volume 'The complete guide to handloading'.

[Nobade]

I had a lot of trouble with that in my CZ 7.62x39 rifle when I was trying to shoot light weight boolits with light charges of fast pistol powder. I wanted to duplicate 32-20 loads. After two firings I started getting misfires - light firing pin hits. I started measuring the headspace on the cases and found them to be very short. So at least on this case, with its small shoulder and large primer, that is a very real problem. Now I make sure I keep the pressures up and save those very light loads for the 30-30.

[ShooterAZ]

Yes, I understand rimmed cases such as the 30-30, 7.62x54R, 303 Brit don't have this issue. They all headspace on the rim, not the shoulder. I am getting a little primer setback too...so I will keep any eye on this. I have a ton of 30-06 & 308 brass, but I don't want to get a mix-up with cases with the enlarged flash holes and the regular ones!

[Wolfer]

I don't think your load will cause setback. I believe the cat sneeze loads are where this is a concern. I understand that drilling your primer holes out is a dangerous practice unless you have a way of positively identifying them.

[Larry Gibson]

Generally it is the "cat's sneeze" loads that do cause this in rimless cases. How many firings depend on a hole lot of things like fit of case body to chamber, taper of case, hardness of the case in the shoulder area, angle of the case shoulder, whether push or CF, etc, etc., ad nauseum. It also will occur in loads where the pressure created by the powder is not sufficient to over come the blast of the primer and force the case back against the bolt face. With regular cast bullet loads in the 1500 fps - whatever velocity it does not seem to be a problem.

It also is usually not a problem with CF actions as the shoulder of the case can only be set back so far before the case rim is held by the extractor and the case can not get blown any farther forward. This is usuall with in the range of the firing pin protrusion so misfires usually don't occur and the shooter is unaware that the problem exists.

With push feeds having a bolt face plunger type ejector is where misfires usually occur. No only does the ejector push forward on the case but the primer force blows it forward also. Eventually a misfire can/will occur and/or the extractor will not snap over the rime and the case will not extract.

I do use rimless cases that take LR primers that have the flash holes drilled out with a #30- 28 size drill. This completely eliminates the problem, especially with the cat's sneeze loads of which I shoot a lot. The larger flash holes also make ignition of fast burning powders such as Bullseye and Unique (sans any filler or wad) in the larger cases such as the .308W to the '06 in case capacity very uniform. It also makes "powder position" almost non existent with such fast burning powders.

I have also velocity and pressure tested those cases (.308W, '06, 7.75 Argentine and 8x57) with the flasholes drilled with my standard loads of 177 - 220 gr cast bullets at velocities of 1850 - 2200 fps with 4895 + a dacron filler. Contrary to opinion there was no increase in pressure. The time pressure curve was a little quicker but the peak pressures were esentially the same. If we stop and consider that we are not adding any additional fuel into the cartridge this only makes sense. Also noted was the slight decrease of the SDs and the ESs of the test strings with the flash holes drilled.

I have found no evidence supporting the theory that using cases with the flash holes drilled to #28 drill size (LR primers) would be "dangerous" with standard level cast bullet loads. Thus I shall endeavor to continue to use them for my "cat's sneeze" level loads in rimless cartridges using LR primers.

I also have tested the .308W cases with the flash holes drilled upward of 55,000 psi with not aparent enlargening of the flash hole or any other problems. However, that test was not sufficient enough for me to say it's ok to use such cases with standard high pressure loads.

Larry Gibson

[Wolfer]

Larry
Extremely interesting, kinda shoots down a lot of the myths I've heard. I assume you try to keep your cases separate so how do you mark them?

[runfiverun]

magic markers if you mix them.
i keep stuff like that in factory boxes i find at the range.

[Larry Gibson]

I do try to keep them seperate and used to mark the bottom with a magic marker also but don't any more. I mostly keep them seperate by headstamp. However, my habit is to inspect the cases after deprimg and or sizing for a defective case (neck split, incipient head seperation, etc.) and visually look through the flash hole for any debri in the case. The drilled out flash hole is readily apparent if one gets in with other cases.

But, as I said earlier, I've pressure tested and found not difference , at least with regular cast bullet loads.

Larry Gibson

[ShooterAZ]

Larry,

Thanks for all the helpful info. I'm not going to worry too much about it right now, as I am not shooting any "cat sneeze" loads at the moment.

[Char-Gar]

We have been through this several times over the years. Very light gallery type loads will cause shoulder set back, but most of us shoot in the 1.4K fps and higher velocities and it is not a problem here.

I have a lot of LC National Match cases I drilled out the flash holes. There is no increase or decrese in accuracy with these cases and 1.4 to 2.0 K fps loads. I took some red finger nail enamel and filled the marking on the case head. A swipe accross some very fine emery paper removed all but that in the markings.

You most certainly do not want to fire these cases with full snort loads.

[gabe123]

Shooter, how are you sizing these cases? Full length or neck size? I find that if you neck size the case, there is no problem with shoulder set back. How far are the boolits seated away from the lands? I think you should address these issues first before contemplating drilling primer pockets, which I find unnecessary.