6 ARC Gas Guns

[Jdowns0415]

Here's the external diameters of a case from each brand of brass fired in each barrel, along with the internal diameter of the chamber of each barrel. Note that the Starline brass clearly is getting pushed into the region at 0.20"-0.23" on the AR barrel where the case web has stopped but the chamber hasn't yet come in to support the case. In that region there's just a ~0.035" brass hoop holding in 52kpsi

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Charting each brand between the two chambers is also instructive. The extra bulge in the Starline AR brass vs bolt action is particularly notable. The Hornady brass avoids the same effect, clearly the increased web height, wall thickness, and radius between the two is putting in work.

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The pricey brass options both appear to do a good job of avoiding getting overly expanded into the AR chamber lead-in chamfer. On the Alpha the web and radius stop at 0.20" height so it's resisting this by wall thickness and brass strength. The Peterson web-to-wall radius goes up to 2.50" so there's a lot more brass mass in the region than on the Alpha.

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Overall takeaways for me from this is that the Starline and Alpha brass are really built for bolt guns. The Alpha seems strong enough to deal with the unsupported chamber region on an AR, but it's depending on an extra 0.005"-0.007" of wall thickness and superior material strength vs the Starline to achieve that, whereas the Hornady and Peterson brass have internal geometry (web height and web-to-wall radius) that appears specifically designed to work in an AR chamber.

Next time, sizing dies and sizing results.

I concur with this based on my experience that after 10 firings every starline piece I have belted to the point of being unusable and my hornady is beat up but still not belted. Sgt of arms belt buster die and hornady watch grade 6mm arc die. Annealed every firing. Using LVR and TAC at max book loads for 105-110gr bullets CCI 450 or rem 7 1/2.

 

[OpticsNerd]

I concur with this based on my experience that after 10 firings every starline piece I have belted to the point of being unusable and my hornady is beat up but still not belted. Sgt of arms belt buster die and hornady watch grade 6mm arc die. Annealed every firing. Using LVR and TAC at max book loads for 105-110gr bullets CCI 450 or rem 7 1/2.

You might find this next set of results particularly interesting.

I found that the Sgt. of Arms belt-buster die did some weird stuff, especially with Starline brass. After some experimenting I ended up using the belt buster as a pass-through die. This process is similar to bullet sizing, the goal is the same as rollsizing. I opened up the shell holder from a Redding G-Rx 40S&W/10mm passthrough base-sizing die set a little so it would hold an ARC case and mounted the belt buster die high enough in the press to get it to work. The G-Rx die has the sizing ring near the top of the die, but the belt buster die has it at the bottom, thus the need for weird mounting. Works though, and the sized case just sits in the die when you raise the press handle. I grab it when the next case pushes it up.

Okay, so what does it do? In the charts below, the yellow "SAAMI max" line is the maximum cartridge diameter dimension from the SAAMI print. The goal of sizing is the get the diameter of the case to be below this line, Up to 0.008" is within tolerance but anything below it is acceptable. If you want to work your brass the minimal amount and have the cartridge sit as tightly as it can (for potentially increased precision) while being in-spec, you want to be just under that line.

This shows the results of using just the belt buster as intended (bottoming out on a normal shell holder, "BB") and using the belt buster as a pass through die in the rig above ("BBPT), along with Starline brass when new and after firing in the AR:

Starline brass, fired in this barrel, expands well above of the spec diameter at a height well below where the belt buster can fully size when bottomed against a shell holder. The result is that used this way, on this brass, it leaves a belt at the bottom. Using it as a pass-through die gets the entire region under 0.33" under the spec diameter.

As a worst-case scenario for what happens when sizing this brass in an RCBS small-base full-length die, vs. the belt buster then RCBS die, vs. belt buster passthrough then RCBS.

Everything is in-spec, but the The BB+RCBS flow creates a pronounced bulge right at the top of the extraction groove. After several firings, this can grow until the rounds won't chamber any more. I don't use the BB die without pass-through anymore.

That said, the RCBS die is clearly sizing the brass much more than is needed to stay in-spec. Here's the results of sizing with just a full-length die and the BB pass through die + a full length die for Lee die and a Mighty Armory die:

both of these dies size to just under the maximum diameter, working the brass much less than the RCBS die. The BBPT+full length die still eliminates the minor belt at 0.2"-0.25".

Here's the full-length die only results from all three dies:

And for the BBPT+full-length die:

For using Starline in an AR, I prefer the BBPT+Mighty Armory setup with the BBPT+Lee in close second. As much as I want to run the Peterson all the time for its strength and consistency, losing pieces at gas gun matches makes me wince. This reloading flow seems to be a good way so far to use Starline in a more or less sustainable manner.

I gotta say that I've been impressed with the Lee sizing die for the money. Compared to almost everyone else they FEEL cheap and rough and almost unfinished. But sizing happens on the inside, and that surface is polished very nicely and dimensioned nearly optimally. Similarly, the expander mandrel is straight, smooth, strong, and well-polished. It's held in with a downright cheap-feeling collet screw, but it does the job.

Final note, the Hornady die has very similar sizing results to the RCBS one. I don't have that one anymore to get into this data set, but some of my older measurements definitely show it behaving like the RCBS die.