10mm AR

[MachIVshooter]

Well, I finally found my 10mm mag brass (buried in a box in the shed where it never should have been!), so I reamed the chamber deeper, loaded up a few rounds, and gave it a go. All I can say is holy fireball batman! 3 small batches, the first 180 gr. FMJ over 18.0 grs H-110. They cycled the action fine, lock on empty no problem! I did five more 180s at 19.0 grs. H110 and ten 200 gr. Hornady XTPs @ 19.0 grs. H-110. With some tweaking of the feed ramp in the extension and a little chamfering of the chamber mouth, the rounds feed from a standard 5.56 P-Mag in semi-auto. I did have to completely eliminate the bottom lug in the extension, but 10mm mag has lower bolt thrust than 5.56mm, so I don't think it'll be an issue.

So, we have determined that 10mm magnum uppers are a very feasible thing, and with the big bonus of not needing special mags like many other pistol calibers or larger bore wildcats.

I'll do a video soon, and then I need to get out with the chrony and do some load development! I've love to hit 1,800 FPS with those 200 gr. XTPs, and I would also like to swage some jackets to make and load 250 gr or heavier subsonic bullets.

The "rails" inside the P-mag are located perfectly for the ogive of 10mm bullets loaded into 10mm mag cases . 10mm case rims are only ~12% larger than 5.56, but the virtually straight case walls do cause the follower to bind with more than 15 rounds loaded, so I will have to make custom mags to have >15 round capacity.

 

[farmerboy78]

NICE!

 

[MachIVshooter]

Got the 416 stainless muzzle brake made and the 12.5" Odin Works Keymod hand guard installed. The brake is 4-1/8" long in order to get the ported portion out past the hand guard, which will partially shroud the suppressor. So the the length from bolt face to front of muzzle brake is 14"

 

[MachIVshooter]

Oh, for those who aren't familiar with the cartridge:

10mm Auto, 10mm Magnum, 5.56mm NATO:

 

[lysanderxiii]

How good are you with math?

I have found that when venturing away from what everyone else has already done, it is best to estimate the expected port pressures, and try figure out how to get around 20 fps bolt velocity before making chips.

 

[MachIVshooter]

How good are you with math?

I have found that when venturing away from what everyone else has already done, it is best to estimate the expected port pressures, and try figure out how to get around 20 fps bolt velocity before making chips.

Decent, but much better at adapting things to work in the physical world.

Pressure should have been sufficient; I believe it to be a gas volume issue more than anything. The 10 mag is cycling it flawlessly with loads that calculate lower pressures than my heavy 10mm loads which failed to lock it open, but are burning 75% more powder at that pressure level.

 

[Prijador]

Wow. That is a kick-butt blaster(!)

 

[barnbwt]

Pressure should have been sufficient; I believe it to be a gas volume issue more than anything. The 10 mag is cycling it flawlessly with loads that calculate lower pressures than my heavy 10mm loads which failed to lock it open, but are burning 75% more powder at that pressure level.

Yup, that's the one drawback with DI in this application; it simply uses more volume to fill up that tube + bolt piston than a long or short stroke piston arrangement. The little 9mm DI guns some builders have made put the gas block right up against the barrel nut, and I've seen AK 9mm gas-guns where the port is right at the chamber leade & a gas tube pipes that pressure forward to the gas block/piston head in order to maximize the time it's under pressure. It's a volume game, and a 10mm bore eats up volume really fast given the volume of powder to work with.

The 9mm DI builds also reminded me of something that may or may not have contributed to the 10mm version short-cycling. Apparently when builders first tried these, they found the AR's button ejector would tilt the 9mm rounds inside the barrel extension (because they are so short compared to 223) enough that they would snag on the extension lugs or even flop out from under the extractor claw. The trick, by a well-known builder, was to press a piece of metal between the lugs where it was catching so the 9mm round cannot rotate off the bolt face as far (SIG actually drilled & pressed a pin ahead of the lugs here for the same reason). I forget if this required knocking off a bolt lug, or only the extractor 'lug.'

My thought is perhaps the 10mm was rotating into the barrel extension recess during extraction --but not as far as 9mm due to its length-- and then dragging over its teeth as it continued back through the extension, re-compressing the ejector button under mechanical disadvantage & considerable friction. Not enough to actually dig into the cases & jam the action as with the 9mm's, but enough to rob precious energy from a balanced equation.

TCB

 

[Buck13]

Too late now, but did you consider trying 2400 with the original chamber? Alliant shows using 1.8 grains more 2400 than Blue Dot with 180s, so should be a little more gas. I wonder if that would have put it over the edge? Not that you would want a gun that only functions with one load.

I made a few rounds of 10 mm with 2400 last summer. Didn't take my chrono to the range that day, so I don't really know what they did. Holes were no farther apart than usual, though.

 

[lysanderxiii]

Decent, but much better at adapting things to work in the physical world.

Pressure should have been sufficient; I believe it to be a gas volume issue more than anything. The 10 mag is cycling it flawlessly with loads that calculate lower pressures than my heavy 10mm loads which failed to lock it open, but are burning 75% more powder at that pressure level.

I'll almost guarantee that if you look at a pressure vs time curve for both, the 10mm Mag will have a higher port pressure.

Pistols are normally very fast propellants, while the peak chamber pressure may be higher it falls off faster as the volume increases due to bullet motion.

 

[MachIVshooter]

I'll almost guarantee that if you look at a pressure vs time curve for both, the 10mm Mag will have a higher port pressure.

Oh, I'm sure it does with the increased volume, but there is a distinct lack of information out there on 10mm magnum! SAAMI doesn't even have a max pressure listing for it.

 

[harrygunner]

Cool project. I feel sorry for people in other countries that can't even think about doing something like this.

Does bring up an interesting question about what bullets to use. Even XTP bullets are max spec'ed at 1200 ft/s for 200gr and 1500 ft/s for 180gr bullets.

But, whatever you hit should be very unhappy. Congrats.

 

[MachIVshooter]

Well, I'm talking to to good folks at Corbin now, gonna gear up to swage 300 grain jacketed hollow points for suppressed use! 300 grain bullets at 1,050 FPS oughtta make a nice, quiet hog thumper that'll put .300 Blk subs to shame

That's gonna be a little bit, though. Not a cheap venture.

 

[LoonWulf]

Always enjoy your builds, looking forward to how those 300s turn out...were you making them from scratch or swaging 41cals?

 

[MachIVshooter]

Always enjoy your builds, looking forward to how those 300s turn out...were you making them from scratch or swaging 41cals?

Gonna go with whatever Steve (or was it Dave? I'm terrible with remembering names) recommends. From our phone conversation, it sounds like swaging down .41 cal jackets will be the way to go. He has uncut ones that are 1-3/8 long, so plenty long enough.

 

[LoonWulf]

Cool, im def looking forward to how this works out, i like the way the 10mag looks already.

 

[MachIVshooter]

Got a little range time in yesterday, did some chronographing. Out of the 10.5" barrel,

180 gr. Hornady HAP JHP, 20.5 grs. H110, CCI 350 primers: 1,670 FPS avg.

200 gr. Hornady XTP, 20.0 grs. H110, CCI 350 primers: 1,630 FPS avg.

SD was about 15 FPS, ES almost 80 FPS

Now we'll start working up hotter loads, see if I can hit that 1,800 mark with 200 grainers Still, though, 1,630 with 200 grain pills is nothing to scoff at. That's about 1,200 ft.lbs., which is KE equal to the fastest .300 Blk loads from 10.5" tubes.

I had also brought the video camera, tripod, and two fully charged camera batteries with me. Unfortunately, the SD card was hanging out on top of my desktop hard drive tower instead of in the camera We'll try again soon.......

 

[Casefull]

Neat project. I have always been a 10 mm fan and have three pistols in that caliber. Talk of 300 grain bullets does make me think of my 458 socom though. Barnes solids would be a good way to go once the velocity gets up there. My casull revolver is quite adept at ripping the jackets off of pistol bullets.

 

[conkli48]

I love the 10mm its a shame the 10 mm magnum never took off. Would be great for hunting and other things. This build is fantastic and i have high hopes for it. I would even like to see a 10mm case lengthend to the full 223 case length for higher velocities. I also would love to see the 10 mm magnum offered in the desert eagle.