M1 Carbine Short 300 AAC/BO

[GrumpyOlGuy]

OCD caused me to try this: 110gr BC=.273. With ~1970fps/960 ft-lbf at the muzzle, gives 9mm parabellum muzzle velocity and energy at ~350+yds (~1200 fps, ~350 ft-lbf; about 100+ yards greater range performance than the standard carbine ballistics). Drops to subsonic at about 400 yds. That's a heck of a pistol round.

Uses 223 parent case, shortened 300 BO dies (to form case), and a 300 BO reamer with a standard barrel. Case is shortened to 1.185in. OAL cartridge is 1.685, so can load into a standard magazine (but only 10 or 11 rounds). Case volume is about the same as the standard, although the bullet seats a little deeper (the bullet is about .125 longer than the standard FMJ). Test results so far (around 50 rnds) -- cycles ok with 12.0 to 13.0 gr WC820; ~2-2.5 MoA. Chronoed at ~1920fps. Haven't tried any higher loadings. My normal carbine load is ~13.2-13.4gr WC820 with a 110gr FMJ. Used a barrel from Sarco. Next up is to try some lil' gun. I'd love to get this up over 1000 ft-lbf and see if the accuracy was ok.

The point was to see if one could extend the effective range of the carbine and stay within the overall cartridge length. And to show what might have been possible in 1940. (Not that the Carbine didn't fulfill its intended objective extremely well. Just could have been a little more with a different bullet). For those of us who really appreciate the way this thing handles. I think 9mm ballistic/power presents a reasonable motivation to keep one's head down.

 

[Corpral_Agarn]

I appreciate this experiment.

Thanks for posting!

 

[LoonWulf]

OCD caused me to try this: 110gr BC=.273. With ~1970fps/960 ft-lbf at the muzzle, gives 9mm parabellum muzzle velocity and energy at ~350+yds (~1200 fps, ~350 ft-lbf; about 100+ yards greater range performance than the standard carbine ballistics). Drops to subsonic at about 400 yds. That's a heck of a pistol round.

Uses 223 parent case, shortened 300 BO dies (to form case), and a 300 BO reamer with a standard barrel. Case is shortened to 1.185in. OAL cartridge is 1.685, so can load into a standard magazine (but only 10 or 11 rounds). Case volume is about the same as the standard, although the bullet seats a little deeper (the bullet is about .125 longer than the standard FMJ). Test results so far (around 50 rnds) -- cycles ok with 12.0 to 13.0 gr WC820; ~2-2.5 MoA. Chronoed at ~1920fps. Haven't tried any higher loadings. My normal carbine load is ~13.2-13.4gr WC820 with a 110gr FMJ. Used a barrel from Sarco. Next up is to try some lil' gun. I'd love to get this up over 1000 ft-lbf and see if the accuracy was ok.

The point was to see if one could extend the effective range of the carbine and stay within the overall cartridge length. And to show what might have been possible in 1940. (Not that the Carbine didn't fulfill its intended objective extremely well. Just could have been a little more with a different bullet). For those of us who really appreciate the way this thing handles. I think 9mm ballistic/power presents a reasonable motivation to keep one's head down.

View attachment 801279

I like stuff like this!

Ive got a couple questions, since im not familiar with how strong the carbines action is.
What sort of pressure can you run the ...uhhhh...300shorty? up to and still be ok in the action?
Would there be any advantage to shortening the standard 30 carbine to be able to use the pointy 110s without them being too long to fit in the magazine correctly?

 

[GrumpyOlGuy]

The M1 carbine is designed to operate at 40,000 psi (to my knowledge). The 300 BO (in its design configuration) operates at higher pressures (upwards of 55,000 to 60,000). The 300BO case volume is quite a bit larger than the standard carbine, and typical loads are higher too, and projectiles can go anywhere from 100gr to 200gr+(subsonic). Compare this to an AR, where the 223 generates around 60,000psi chamber pressure.

The short AAC that I made has approximately the same powder volume as the standard carbine. I don't know if the differences in case/chamber shape would cause any dangerous pressure conditions to develop. Since the powder volumes and loads are the same (out to the third decimal place), I was hoping that the pressures would be very similar. I fired the first 20 or so rounds with an armor blanket over the rifle and examined the cases (primers, necks, etc.) for signs of excess pressure. (I know this is not an exact measure by any means, but I have seen where differences in powders can produce significantly different post-fired primer condition). For loads below about 11 grains or so, cycling deteriorated -- some failures to feed and failures to extract. This indicated insufficient energy/pressure in the gas operating system (and so a lower limit of operating pressure. This was similar to what I've found in the standard carbine loads).

Also, the rounds chrony with very similar velocities compared to standard carbine rounds with similar loads. Then I fired another 40 or so from the shoulder. Recoil is similar (very subjective measure). A standard carbine loaded with 85 grain bullets over 14.5-15.0 grains fires with a more noticeable difference (much, much bigger bang and recoil) than this modified cartridge (even though muzzle energy is lower).

As far as shortening the standard carbine case, the powder volume available would be significantly reduced. With the standard carbine case, the much greater length of bullets with higher ballistic coefficient causes the need for the bullet to be too deeply seated in the case. The bullets I used were about 0.82 inches compared to 0.680 for a standard 110 FMJ (almost an eighth inch). So in order to keep the cartridge at a length to fit the magazine (1.68 inches) the bullet ogive is actually inside the case (aft of the case lip). You can make up individual standard cased rounds (with a longer than standard OAL cartridge and hand load the round into the rifle's chamber) and achieve a similar result (but these won't fit into the magazine).

 

[LoonWulf]

The M1 carbine is designed to operate at 40,000 psi (to my knowledge). The 300 BO (in its design configuration) operates at higher pressures (upwards of 55,000 to 60,000). The 300BO case volume is quite a bit larger than the standard carbine, and typical loads are higher too, and projectiles can go anywhere from 100gr to 200gr+(subsonic). Compare this to an AR, where the 223 generates around 60,000psi chamber pressure.

The short AAC that I made has approximately the same powder volume as the standard carbine. I don't know if the differences in case/chamber shape would cause any dangerous pressure conditions to develop. Since the powder volumes and loads are the same (out to the third decimal place), I was hoping that the pressures would be very similar. I fired the first 20 or so rounds with an armor blanket over the rifle and examined the cases (primers, necks, etc.) for signs of excess pressure. (I know this is not an exact measure by any means, but I have seen where differences in powders can produce significantly different post-fired primer condition). For loads below about 11 grains or so, cycling deteriorated -- some failures to feed and failures to extract. This indicated insufficient energy/pressure in the gas operating system (and so a lower limit of operating pressure. This was similar to what I've found in the standard carbine loads).

Also, the rounds chrony with very similar velocities compared to standard carbine rounds with similar loads. Then I fired another 40 or so from the shoulder. Recoil is similar (very subjective measure). A standard carbine loaded with 85 grain bullets over 14.5-15.0 grains fires with a more noticeable difference (much, much bigger bang and recoil) than this modified cartridge (even though muzzle energy is lower).

As far as shortening the standard carbine case, the powder volume available would be significantly reduced. With the standard carbine case, the much greater length of bullets with higher ballistic coefficient causes the need for the bullet to be too deeply seated in the case. The bullets I used were about 0.82 inches compared to 0.680 for a standard 110 FMJ (almost an eighth inch). So in order to keep the cartridge at a length to fit the magazine (1.68 inches) the bullet ogive is actually inside the case (aft of the case lip). You can make up individual standard cased rounds (with a longer than standard OAL cartridge and hand load the round into the rifle's chamber) and achieve a similar result (but these won't fit into the magazine).

Very cool! Thanks I've shot a bunch of carbines but never worked on one.