6gt ARC CDG and light primer strikes

[_Thor_]

Hi all,

I recently broke in a new 6gt build with an ARC CDG. First 20 shots were all factory Hornady/GAP ELDMs and ran fine. My next twenty were my first reloads in 6 gt. Virgin Alpha brass with CCI 450s, 32.5 gr Varget. About 7/10 in each row failed to fire. Light primer strikes. I seated the primers with the Frankford hand primer, though they tended to catch a little on insertion. I’ve since found that i was using the larger primer keyway in the tool with the small primer “ram” piece. I’m not sure if this issue is more related to primer depth or the incorrect tool configuration or something else. Any advice? I’d like to fix my reliability issue with as few trips back and forth to the range as possible since my opportunity to test is limited.



In the picture, the round on the left did not fire, but the right did. What does the rounding on the left primer indicate?

 

[AllenOne1]

Did you pull the bullet on the non-fired rounds to see if there was powder in the case and if the primer actually fired? What was the sound when you had a non-firing cartridge?

 

[_Thor_]

I haven't pulled the bullet, but I can hear that there is definitely powder in the case. The sound was just the firing pin "tink". Definitely 0 ignition. I also recocked several and tried firing them multiple times, with no luck.

 

[memilanuk]

Did you by chance measure the primer pocket depth before hand? Primer seating depth after seating?

I'm not trying to get you headed down any rabbit hole of measuring/sorting primer seating depth on the regular, but mainly as a troubleshooting tool here.

If the primer pocket is fairly tight - and my limited experience with Alpha OCD brass in .308 Win SRP says 'yes' - then you do need to pay some attention to a) what the primer pocket depth is, b) what the primer height (bottom of cup to top of anvil) is, and c) make sure that you're actually seating the primer far enough in to where the anvil has some pre-load/crush/consolidation (all terms for the same thing, depending on the literature/source) so that it will go off reliably. Otherwise it's entirely possible that all you're doing when you drop the trigger is just seating the primer a little deeper - but not actually crushing the pellet and causing it to go off.

I'm not saying that one can't reliably seat by feel, without checking the numbers... but if you're new enough that you didn't realize you had the wrong primer ram in place, you might not be there yet.

 

[_Thor_]

I did try to take some measurements after the fact using alpha brass from the same lot that I hadn’t loaded.:

Alpha case primer pocket depth: .122 - .124

Cci 450 height: .1205

The above being true, I would think I need to seat them 2 thou short of the end, right? That said, I measured the no-fire and fired cases primer depth to the best of my ability and got 0.000 on both, which puzzles me.

 

[memilanuk]

I'm guessing you're measuring the depth with the tail of your calipers? Not the best tool, but short of spending $$$ on a dedicated device, certainly usable.

Pocket depth (0.124) minus primer height (0.120) gives a difference of 4 thou. So for the primers to be just touching the bottom, they'd need to be 4 thou below flush. Ideally, for a bit of pre-load/crush, you want them 2-3 thou below *that*, so say 6-7 thou below flush (in this example).

 

[_Thor_]

I'm guessing you're measuring the depth with the tail of your calipers? Not the best tool, but short of spending $$$ on a dedicated device, certainly usable.

Correct - I can't say I trust the measurements entirely, especially given the dimple in the primer already, etc.

That's interesting though - I had assumed I had seated too deep, but it sounds like you're saying seating too shallow is actually the more likely issue? And in that case, I'm wondering how best to adjust my Frankford tool. If I just adjust as far out as possible short of creating half-moon imprints in the primer cup, is that ideal? As you said, I can also attempt to measure with calipers, but that's a little spotty.

 

[RegionRat]

Just food for thought...
7 of 10 is a pretty high share of failures. In some ways, that is bad but in a way it should make it easier to isolate the root cause.

If the ammo situation allows it, I would take about 10 to 20 of those factory rounds with me on the next test session. You may find an obvious cause but if something still isn't right, then I would go back the last ones that were still working and verify they still do.

The fishbone diagram for troubleshooting a bolt gun failure can be long, but you can cut it down by splitting the search between the gun and the ammo.

By chance, have you also checked the headspace/shoulder datum length of your virgin loads versus the factory?
How about the shoulder datum length after firing the ones that did ignite?
While primer seating depths can play a role, so does the way the cartridge sits in the chamber. If a case is short and a chamber is deep, we can get problems.

Did you go back to factory ammo and have no failures?
Did you inspect the bolt and firing pin spring?

 

[straightshooter1]

I did try to take some measurements after the fact using alpha brass from the same lot that I hadn’t loaded.:

Alpha case primer pocket depth: .122 - .124

Cci 450 height: .1205

The above being true, I would think I need to seat them 2 thou short of the end, right? That said, I measured the no-fire and fired cases primer depth to the best of my ability and got 0.000 on both, which puzzles me.

My Alpha .308 brass measures a consistent .122" pocket depths and my CCI-450's I use in them measured .121". CCI-450 cup height is ~.113", which means the anvil is protruding above the cup by ~.008" . . . plenty of room for "crush". I typically like ~.003" of "crush", meaning I look to have my 450 primers at .004 below flush (flush being level with the base of the case).

If these 450's are seated flush, then yes, it could cause problem you've had. But you're firing pin protrusion is a factor as well, where if it doesn't protrude enough, light strikes can easily happen. . . especially in combination with primers not seated where they can be struck properly. It would be interesting to see what those fired factory case primers looked like to see the the strike looks like compared to the one's you showed.

Note too that the 450's have a thicker cup than standard (like the 400's are relatively thin), where firing pin protrusion and spring strength can be an issue.

 

[PinesAndProjectiles]

Did you install the barrel?

Is it barrel nut or a shouldered barrel?

How did you set headspace?

 

[LR1845]

Sounds like your trigger may need adjusted I had the same light strike issues…here’s how to make sure it’s adjusted correctly.

 

[_Thor_]

This is a shouldered prefit barrel that I installed. I checked go/no go with Manson gauges and it checked out.

Also, I followed the linked video for the trigger setup.

I didn’t switch back to factory rounds after the reloads, but I would bet that they work fine. It really seems like my reloads are at fault.

Here are some headspace measurements:

Unfired Factory ELDM: 1.3570
Fired factory ELDM: 1.3580
Faulty round: 1.3555
Virgin unloaded brass: 1.3505
Successfully fired reloads: 1.3560

So the fire formed cases are clearly 2-3 thou bigger, but then, some reloads fired with 1.3560. The virgin unloaded brass is even smaller according to my measurements - not sure if seating the bullet should change the way it measures in the comparator. If this is a headspace issue, it begs the question- how are you ever supposed to fire form virgin brass if it’s too short?

The primers on the factory rounds do appear to be 2-4 thou below flush, but this is with a different brand of brass entirely.

When I crank my priming tool to the deepest setting, I can seat 2 thou below flush by really muscling it - this puts an indent on the primer though.

Hoping it comes through in the pictures, but the primers on the faulty ones just really look different to me. Very rounded, like a donut. Whatever that is seems abnormal.

 

[AirgunnerPCP]

I had a similar issue with 6GT Alpha brass. Virgin brass headspace was short so what I do now is jam the bullet ~.010 into the lands. This prevents the case from being pushed forward by the firing pin and I get 100% reliable ignition.

 

[memilanuk]

I think it's fairly typical for bolt actions to have 45-50 thou of firing pin protrusion. The headspace would have to be off a *lot* for the firing pin to not fully crush the pellet in the primer and ignite the powder column. @RegionRat might have better hard numbers on that.

 

[spife7980]

Hoping it comes through in the pictures, but the primers on the faulty ones just really look different to me. Very rounded, like a donut. Whatever that is seems abnormal.

It’s because they didnt make 50-60k psi blowing up to force the material back against the bolt face. You just have a little dimple from the firing pin.

The fired brass being shorter is normal, it will fill the chamber better in subsequent firings. It’s not just getting bigger length wise, it’s swelling up in diameters too. That swelling to fill the cylinder pulls some of that material back a bit, hence the not fully grown in length.


It’s not just the empty chamber space that the excess firing pin protrusion has to overcome, it’s the cartridge moving in that space over time, and it’s mass absorbing the impact force like a shock or strut, that lessens the impact on the primer. Impact being needed for it to go off.

So it important that it doesn’t have too much free space in the chamber for the case to slide forward into, off of the bolt face. It’s also important that the primer is seated to the bottom of its pocket so it also doesn’t absorb and spread the impact force over space when the firing pin seating it fully to the bottom of the pocket.

 

[_Thor_]

I had a similar issue with 6GT Alpha brass. Virgin brass headspace was short so what I do now is jam the bullet ~.010 into the lands. This prevents the case from being pushed forward by the firing pin and I get 100% reliable ignition.

So do you have to kindof "waste" the first firing of every piece of virgin brass then? I'd imagine that would be different than what you'd do subsequentl for load developmenty, so you wouldn't want to use those jammed loads during, say, a match. Do you load lighter to just get the fire-forming out of the way?

It’s because they didnt make 50-60k psi blowing up to force the material back against the bolt face. You just have a little dimple from the firing pin.

The fired brass being shorter is normal, it will fill the chamber better in subsequent firings. It’s not just getting bigger length wise, it’s swelling up in diameters too. That swelling to fill the cylinder pulls some of that material back a bit, hence the not fully grown in length.


It’s not just the empty chamber space that the excess firing pin protrusion has to overcome, it’s the cartridge moving in that space over time, and it’s mass absorbing the impact force like a shock or strut, that lessens the impact on the primer. Impact being needed for it to go off.

So it important that it doesn’t have too much free space in the chamber for the case to slide forward into, off of the bolt face. It’s also important that the primer is seated to the bottom of its pocket so it also doesn’t absorb and spread the impact force over space when the firing pin seating it fully to the bottom of the pocket.

I'll definitely try seating the primers as deep as they'll go on the next batch. I'm just also trying to reconcile what the process should be for virgin brass. As I said above, it seems like virgin brass can't be used for regular load development - you have to waste a firing to get it "usable" - unless you just run bullets jammed into the lands all the time?

I'm also curious why I had any of them go off at all if the headspace is the issue. I measured the virgin brass and it's all very consistent.

 

[AirgunnerPCP]

So do you have to kindof "waste" the first firing of every piece of virgin brass then? I'd imagine that would be different than what you'd do subsequentl for load developmenty, so you wouldn't want to use those jammed loads during, say, a match. Do you load lighter to just get the fire-forming out of the way?

No not a waste at all. Typically the first firing of brass coincides with a fresh barrel. What I do is load the 200pc of brass (I typically only get 200 per barrel) jammed with a known, lower charge weight load that typically ends up around 2780ish with 107 SMKs. I'll shoot the first 20 to get data and ensure its about where I expect it. Then I'll use the next 160ish to train with and finish the last 20ish to check speeds again. At that point the fresh barrel is broken in and virgin brass has all been fired.

With this method, the accuracy has always been more than good enough to shoot a match with. However I like to minimize variables for matches so I use the first firing for positional training.

I will say that load development for me is mostly just checking velocity at different charge weights and choosing whatever charge yields whatever speed I want. Groups have always been around 1/2 MOA.