Primer seating depth effects on velocity spreads

[NighthawkCG]

Looking for some real world experience while I wait for equipment to test this. I have been using the a dillon 550 for primer seating primers in .223. I loaded some plinking 55gr rounds and noticed that some of the primers felt just flush if not a hair high so I started measuring them. Brass was all matched headstand (LC13), swaged upon first firing and has had anywhere from 3-6 firings. I never uniformed the pockets and when running in progressive, haven't really cleaned the pockets either.

I switched to some brass that I use for long range that does have cleaned pockets (but not uniformed) and the primers were below flush, but only by about .001-.002. They are relatively consistent, but vary by about that .001-.002 as well (as best as I can measure). Ideally I've read that .004 is the goal. I did a bit of the primer cup modification to the press and took down .002, to get deeper, but don't want to overdue it before looking at the brass.

My question is - How much does inconsistent primer depths effect velocity spreads? I ask because I have been struggling with high ES/SD on all of my .223 loads and seem to have plateaued with a mid to low teens SD on the loads. Could uniforming the pocket and flash hole be the ticket to get those spreads down to around 10? Anyone here ever test the effects on consistency when uniforming the pocket and flash hole?

I know the dangers of seating high and seating too low, but I wondering on the effects on ignition assuming there are no safety/reliability issues. If my primers are seated all of the way in the pocket, at .001 or .002 (with some inconsistency), is there a benefit to uniforming, seating to .004ish? If so, how much (general answer is good, don't need precise numbers)? I plan to test this all out, but am looking for some of the knowledge and experience from you all as well.

Thanks in advance.

 

[Supersubes]

Ive never read or heard of the .004 recess number before. The tolerance for firing pin protrusion for a bolt action is about double that. .004 sounds totally arbitrary to me. There are several manufacturing tolerances at play here. Variances in pocket depth, the floor and wall shape, and diameter control how deep the primer can go. Primers themselves vary in those same ways. Recess under the case head is simply a product of all those things, along with how hard you cram it. I prefer priming with a hand tool, for feel purposes. I seat to the bottom of the pocket, and then just a smidge more. Bench rest shooters used to call this “activating” the primer. Obviously the primer will go bang without this, but the idea is to get the cup to squeeze the priming mixture against the anvil just slightly. Hard thing to measure of course, which is why hand priming tools have a lot of leverage for good feel. Note though that the anvil feet protrude above the edge of the primer cup. The anvil feet should be touching the bottom of the pocket.

I think all the pocket uniforming and cleaning is a feel good step, and I don’t do any of it. Providing you have centered and burr free flash holes, priming is perhaps the least important factor. Occasionally you can run into a load combo that just isn’t happy. Certain cartridge/powder/primer combos for whatever reason just dont work well. Before you go through all the trouble of uniforming stuff, id recommend a primer switch as the easiest test. More than likely though, the obvious stuff is causing your sd’s. Neck tension, neck finish, charge weight variances, case capacity variances, etc. Is this in a bolt gun or AR? Are you loading progressively?

 

[padom]

The .004" recess number is actually what AMU uses as they say between .003" - .005" is what they look for and consider anything outside these numbers out of spec. This ensures mostly safety but certainly has a play in SD's as well. Primers should always be slightly recessed and not flush and especially not raised above the case head.

I have my primers set for a very slight crush which always leaves the primer recessed in the .003-.005 range. You can measure this with a set of calipers.

http://bulletin.accurateshooter.com/2016/01/primer-seating-depth-uniformity-and-accuracy/

 

[GhengisAhn175]

You’re struggling with high ES and SD because you didn’t properly develop a load for your system. Primer seat depth doesn’t have a major effect to what you’re seeing.

Do proper load development and those numbers will shrink. I suggest you lay out your procedure in depth so we can tweak your loading regimen

 

[tscoz]

I use a primer pocket uniformer each loading but after the first time it only serves as a primer pocket cleaner. I’ve never cleaned primer pockets for any reason other than to give me a consistent feel when seating primers. It might provide an incremental improvement in SD/ES but certainly not enough to yield the results that you’re looking for.

I completely agree with GhengisAhn175. If you work on developing a load and on your reloading procedures you'll likely see a much bigger improvement.