Base size charge vs "normal" size charge

[StoneHendge]

My new RCBS Base Size dies are showing up today to replace my Hornady's. I went with them since reliable feeding is my primary objective (one bad feed can ruin a stage and hence your day!). Plus Larry is blowing them out the door for $27 a set. Worth a try at that price before going "boutique"

I recall from years ago that my velocity went up going from virgin brass (which is base sized) to normal full sized brass and it was a noticable amount. I'm going to tweak my load and if I assume the same velocity is going to me the same accuracy, I should increase the charge slightly. 

I know - seems counterintuitive. Anyone have recent experience in any caliber? Or among those who did load development with virgin brass and are now on fired "normal" sized brass.

[trianglevelvet]

I'm guessing you're talking about small base dies.

The usual way that people make the decision on whether they're needed is by sizing some brass and seeing how it chambers.

A standard way to do this is to start with some factory ammo.
This will be sized to SAAMI specs. Fire some in your gun and collect the brass.

Use a headspace comparator to measure some of  the fired brass.
Write down the measurement.

If its a bolt gun you usually want to size your brass "0.002 shorter than the fired size.
For a gas gun you'll be looking to size it "0.003-" 0.005 shorter than fired size.

Test your sized brass to see if it chambers smoothly in your gun.

Most of the time as long as you're full length sizing you won't need a small base die. 

The reason small base dies were invented is because people had the idea back in yesteryear that neck sizing might be the key to accuracy.
After a few firings the body of the brass would no longer fit the chamber.
And if it was full length sized after that it would still be a tight fit since the brass was springy and work hardened and would spring back.
So small base dies were introduced to help get the brass back to a decent size.

If your chamber is made to spec and your dies are made to spec and you full length size after every firing you'll likely never need a small base die.



Now the other question:
I think you're asking if using a small base die will decrease your case capacity and increase pressure.

Yes.

How much?

No way to know without measurement of case capacity between brass sized in each die.

If you switch to using a small base die its probably good idea to test charge weights starting low and working up,.

This difference is probably very small , but if you're working near max charges it could be enough to make a real difference.

[Keycap]

Use a headspace comparator to measure some of  the fired brass.
Write down the measurement.

If its a bolt gun you usually want to size your brass "0.002 shorter than the fired size.
For a gas gun you'll be looking to size it "0.003-" 0.005 shorter than fired size.

Test your sized brass to see if it chambers smoothly in your gun.

Is there a specific way to test to see if reloaded rounds chamber smoothly in a gas gun? I know it sounds like a stupid question, but what are we looking for when firing reloaded brass that is .003 shorter than the fired size?

[Thelaststraw]

My understanding of the minimal shoulder bump is to reduce unnecessary sizing of your brass. By fully sizing to SAAMI rather than a couple thou can extend the life expectancy of the brass. The reason for sizing shorter in gassers is for reliability.

Two ways to check that I know of with a gas gun....one is at the loading bench with a dummy and the other is anywhere you are comfortable racking one in. I don't worry too much about getting a few extra loads from my brass for this cartridge, brass isn't plentiful currently but 6ARC is far from boutique status and relatively available. If I had to put in a bunch of time like neck turning and stuff, maybe so, but for me I just bump back 0.005 ish and run it.

[StoneHendge]

My issue has been getting rounds caught up with the rifle on a bag on a barricade with the mag pressed pretty hard against my bag, which ends up tilting the rounds down slightly and then they get caught up on the barrel extension feeding.

   

It's happened 3 times with about 1,000 total rounds in the barrel. One was in a match and I managed to clear it pretty quickly, one was in practice, and the last was early in a match and I needed to crater while on the clock and didn't have enough time to finish the easiest stage of the match. That was the one that ruined my day.

A tight BCG channel in a billet receiver is the likely main culprit (no ability to "wiggle" when the round got caught up) and I've moved the barrel to a different set up that should be a little more forgiving. Since my Hornady sizing die is now in a landfill, I figured I'd go the small base route because having slightly narrower brass will also give it more "wiggle" while feeding. I've never been one to put much into the overworking the brass argument since primer pockets are almost always the first thing to go. But I'll accept shorter brass life (if there is indeed any) and deal with the more frequent trimming for this application.

I'll note that I haven't had this particular problem in any other caliber and presume it's partially due to the amount of bullet forward of the neck as well as the amount and angle of the shoulder. Yes, it could also be my barrel extension, but that gets complicated. My goal is to simply have it never happen again

Loaded some up a few tenths of a grain on either side of where I had been loading. It'll be interesting to see where the velocity ends up.

[r.tenorio671]

.... FWIW, it sounds more like your technique of pressing your mag against the bag (or barricade) that causes your stoppages. It would be possible to put a stop on the rail forward enough to prevent mag tilt from pressure on its front or train to avoid using the mag as a hard stop in that kind of scenario.

[Thelaststraw]

Maybe spreading or bending the ends of the lips of your magazines could counter the nosedive. I've seen that done with some cartridge/magazine combos on bolt guns. What bullet(s) is this happening with? Just curious.

[StoneHendge]

.... FWIW, it sounds more like your technique of pressing your mag against the bag (or barricade)  that causes your stoppages.  It would be possible to put a stop on the rail forward enough to prevent mag tilt from pressure on its front or train to avoid using the mag as a hard stop in that kind of scenario.

It's not a technique, it's simply the product of certain barricades and angles which in my case, was a motorcycle seat in a reverse kneel for 5 targets and then a deployed bipod from prone for the 5 targets. A rifle should feed at any position the mag can be in in the well.  Monopodding would resulting the same top forward / downward tilt position.

[StoneHendge]

I had been running 2446 with 105 RDFs with 26.8 gr of Varget in a Hornady die sized Starline 6.5G case (16" Lilja).

Multitasking since I put the barrel on a different build and need to zero the scope, I strapped on Magnetto and started at 25 yards with a just cleaned bore. First three at 26.5 gr were 2453 sd 3 es 7 and touching (albeit at 25 yards). I pretty much called it a wrap with the testing at that point given the few extra fps a just cleaned bore gives. Made a windage adjustment and did the 3 at 26.6 which were a few fps faster (didn't write it down) just to have it ready for 50 yards next outing. Off to the puller for the remainder.....

I'm pretty surprised that it was as much as a 0.3 gr difference, but hey, that's 3 more rounds per pound! Good data point for anyone doing load development with virgin brass for when they switch to fired brass.

[StoneHendge]

Another jam in a practice session today. Was using a 25 round ASC Grendel mag instead of my normal 20 round Duramags to see if there would be a difference with less forward mag body curve. Had my bag on top of a very solid and stable prop and really pushed hard in to it on purpose to see if it would feed. Had one in the chamber as I set up, took the first shot and the tip of the 105 RDF got pressed hard into the base of the barrel extension during the feed. Couldn't finger it out as the base was under the mag lip. Took 2 hands to remove the mag and the bullet tip was pretty mangled. I was doing a drill with 3 targets and three props and this was on the third prop and I wouldn't have been able to recover from it before timing out. I'll attribute this one to the ASC mag as I've never had one of their Grendel mags function properly, but still a bit disconcerting. My 6.5 CM with a 20 round Duramag and a .223 with a 20 round PMAG handled it fine. Beginning to wonder if it's my barrel extension or a quirk of the caliber. And wondering what mags the DoD entity uses.

[r.tenorio671]

 And wondering what mags the DoD entity uses.

...an interesting question for sure, leads to other questions about DoD's bullet choice (secant, tangent, hybrid), COAL of DoD's prescribed load, etc. and how it affects feeding.  Good info on how specific brand of mags were affected by your testing.