just another 300 cartridge

[pug]

I was curious if there was any interest in another 308/223 based cartridge for the AR-15 or any other .223 platform. Not really for subsonic use because of increased case capacity but suited ideally for (supersonic) 125 to 150 gr bullets yet all the heavys can be used as well. Calculated power shows higher than 7.69x39 for same bullet weight but that can be off somewhat. This isn't a money maker because I don't make barrels for sale. I am just wondering what everyone thought.

[Spook]

Pug , I've actually formed up a few 30 TCU's...I was thinking the old 147grain steel core milsurp for an AR -pistol or SBR ,what shoulder angle were you looking at? There's a load a Reloadersnest for 150's at just over 2100 FPS!
The 7mm TCU was always pretty accurate even at distance out of TC's.

[pug]

Spook you are talking about a whole lot more case capacity than I am. The guy at the shop shoots TCU and the cases are a little long for what I had in mind. I was looking more at a 30 degree shoulder and case capacity just shy of 25 grains. In order for the cartridge to run fairly smooth at full auto rates the cartridges must stack correctly in the magazine and that is why I think I would go with a shorter case. As far as velocity I am not going to guess although the computer that doesn't always know what it is talking about says over 2200 fps for a safe load out of a 16" barrel. It also says pressures can go up to a max of 70,000 psi so I don't think there is a problem of not enough pressure. I got the cartridges made up and most of the specs filled out but I may still lengthen the neck.

You know I looked at over 75 different 308 cartridges and still didn't see the 30 TCU which I knew about. I was also surprised to see the 30-223 had less case capacity than the 300/221.

Not sure this was the right place to post this subject so forgive me if I made a booboo.

[pug]

Spook here is a picture of the cartridge with a 150gr FMJBT seated to 2.200" compared to a 300/221 on the left. I was initially testing mags with magpul followers and everything stacks fine but green GIs cause a little more drag.

[Spook]

Pug, I cant see a pic?

[320pf]

Pug,

I would like to see the picture. Somehow the picture did not come throught with your post.

320pf

[pug]

I will see what I can do but it may be beyond my skills. There is a thumbnail pic of the two cartridges side by side in the post but perhaps it is local on just my machine.

Removed the attachment and uploaded it again. Let me know if it still isn't there. The reamer should be here in a few weeks so I should know a little more about the performance.

[320pf]

Pug,

Cool!! It looks like you can rechamber an existing 300-221 fireball barrel for the new round... Am I correct?

If it works I would defiantly like one :)

So what are the actual dimensions? It looks like the case is about 1.5". It looks like there might not be enough shoulder to get reliable headspacing. However, I assume that you blow out the case and increase the shoulder. An honest 2150-2250 fps velocity with a 150 gr bullet would be just the ticket for major rifle!!!! Very interesting!!!!

320pf

[kurtz]

How do the dimensions compare to the HK 7.62x37mm, see pic

http://www.quarterbore.net/forums/ph...to/335/cat/535

[pug]

kurtz I am very glad you showed me that. Even though I didn't see that particular one in any of the databases I couldn't understand why know one had come up with it because it seemed logical. After entering the dimensions for that size cartridge I found very little performance increase over the 300/221 using ballistics software so I changed the dimensions and bumped up the case capacity until I saw a reasonable increase. Then I tried to form up the case and it seemed to work out so off everything goes to have the reamer made. We shall see and I know for a fact these things don't always work out as we expect them to. There are details missing both in picture and data but I hope everyone understands I would like to see how it works out first.

320pf yes as far as rechambering but I am going with a smaller neck diameter than 300 fireball initially. There will most likely be a small step in the neck diameter right above the shoulder if a 300 fireball chamber is recut to the new dimensions. With the neck I specified I do not seem to experience the big first round drop in velocity and a lot of the loads have standard deviations under 10.