Primer Seating Depth Variation

[bbowles]

I am reloading Lapua brass for .223 (5.56 chamber in an AR). I am using a RCBS hand primer tool (one with round tray & uses shellholder, not the universal one).

Question I have is when I seat primers I can feel them seat and yet if want to can squeeze harder. I usually try to stop when bottoms out and squeeze slightly more for good measure. What bothers me is I use my calipers to measure seating depth and I get WIDE variation no matter how consistent I try to be. From .0005 to .008 but most do average in the .003 - .005 like I think they should. Just don't like the few that are hardly below flush to ones seating more than .005.

Am I doing this wrong? Tool not right? Or is this just way it is. Hate to check every primed case! But feel I need to and try to carefully seat further the ones that are not at least close to .003. Thanks for the help!

 

[MikeInUtah]

I always rotate the case a little once I feel it seat initially for a final squeeze. This is just to make sure the primer is level. I also an not too concerned with the depth to the point I am using calipers to confirm which percise depth the primer is at. A quick feel is good enough for me.

 

[jagged77]

Lapua brass should be pretty consistent and shouldn't need uniforming. Try measuring some pockets, the measure your primers then finally measure your seat depth. If the numbers line up before you seat then it's something you are doing, like crushing the primers in with that last squeeze.

I will point out that providing the primers are of a safe depth and not crushed, it shouldn't make much of a difference if there is a slight variation. Can you shoot the difference? I wouldn't worry about it but if it does bother you then go ahead and get a uniforming tool and seat primers with a precision tool.

 

[bbowles]

jagged77,
I do uniform all my primer pockets whether they need or not. I do not squeeze hard once I feel them hit the bottom. I have not measured the pockets though since I figured Lapua brass ought to be good.

I, like you, doubt that I could tell the difference in groups but worry with the few that are barely below surface being safe (about .0005 to .0015)? As long as I feel the bottom and they are at least barely below head can I assume they are safe?

Thanks to you and mmonten1 for your advice!

 

[afate45]

I've loaded 1000s of rounds for my AR using cases I primed with the same RCBS tool. Don't worry about it, as long as they are below flush, you should be fine. Like mmonten said, rotate the case for the final squeeze and that will give you a little bit more depth. I do the same thing for my AR.

 

[bbowles]

Sounds good! I was trying to get all the accuracy I can get out of it for semi long range. I primarily use this rifle for long range varmint and target shooting. So primer depth won't affect accuracy much at all?

 

[bigedp51]

Below is a illustration of seating a primer with a slight crush, if you look at your primers after seating with a magnifying glass you will be able to see if you are flattening and over compressing the primer.

Also when seating your primers the inside of the rim is where the case is resting in the shell holder, if you have dings on the inside of the rim this can affect primer depth. (on adjustable units)

Also if you have any defects or raised areas on the base of the case this will effect your primer depth reading. I measure the depth several times when using vernier calipers, because if it is tilted your readings will vary.

As stated above if a primer pocket uniformer is used all the primer pockets will be the same depth. Also checking and measuring your primer height and checking for height variations on your primers might solve your problem.

We live in a plus and minus world.

Mil-spec 5.56 case dimension requirements, primer pocket depth can be .118 to .122 during manufacture.

SAAMI primer pocket dimensions, commercial cases can be .117 to .123

I do not have the print for the military 5.56 but for information purposes the 7.62 primer is to be .008 below the surface of the base to help prevent slam fires.

 

[bbowles]

Great stuff bigedp51!!

Guess I assumed with Lapua brass and CCI BR4 primers I would not have much variance but probably not. I will check for dings on rims as these are twice shot brass and do have dings. However, thinking back when brass was new they still had these large seating variances with same primers.

Guess my main concern is safety. Making sure primers seated at least below flush and not crushing them either. Dumb question--If I crushed one will they just not fire or worse?

Thanks again for your excellent pics and explanation.

 

[bigedp51]

bbowles

What is strange about your reply is the CCI BR4 primers should have the least or less primer height variations than standard grade primers. Meaning more quality control and uniformity goes into BR4 primers than their standard grade primers. I find measuring primer pocket depth and primer seating depth hard to do uniformly with my vernier calipers due to tilting of the calipers. I drink too much coffee and have chronologically gifted eyesight and I always seem to tilt the calipers getting false readings.

The military went to thicker primer cup standards and made the M16 firing pin lighter to reduce the chances of a slam fire. The majority of slam fires with the M14 and M16 rifles happen when a single round is loaded "WITHOUT" the magazine in place to slow down bolt velocity. With the thicker primer cups (.025) and lighter firing pins the slam fire probability was reduced to one and ten million chance of having a slam fire. This goes up greatly with reloaded ammunition and progressive presses and not inspecting each primer for proper seating. Meaning most slam fires are caused by sloppy reloading practices and not paying attention to your work.

In 46 years of reloading I have never had a slam fire with reloaded ammunition, BUT I did have a factory loaded ammunition slam fire with a SKS that had a small sliver of brass jamming the firing pin. (This also required a change in underwear) ;-)

 

[M40_A1]

I uniform .308 Lapua primer pockets because I found the uniformer cuts some and doesn't touch others. Lapua may be MORE uniform than other brands, but it's not perfectly uniform. What's interesting is that since many of the pockets are not being cut at all by the tool, my "uniformed" pockets are STILL not perfectly uniform.

 

[Mad_Charlie]

I use a variety of cases in an M1a and a 700, I can't shoot the difference between uniformed and un-uniformed primer pockets but I uniform all of them to make sure I don't get a "shallow" pocket or a "high" primer.

Same thing for my AR15's.

Just part of case prep to me.

Like M40_A1, some cut, some don't.