Current household inventory includes a 5.56 RRA M4gery and 5.56 RRA pistol. My teenage son got a stripped DPMS receiver for Christmas. So, we now have the option of buying/assembling the next upper in either 5.56 or something else, which may or may not include my 5.56 carbine upper moving to the new lower. [Weaving our way carefully thru all of the âconstructive manufacture of an SBRâ BS.] I do not hunt; the purpose of the new version of my carbine will be SD. Tell me why I should go out on the 300 BLK (or other) âshiny new thingâ limb, instead of buying 62 gr M855 and maybe reloading to 68gr for another 5.56. My 5.56 ammo inventory is âsufficient for my needsâ, as they say. Another caliber requires me to reload rifle rounds in addition to my handgun reloading. Iâve gone down the route of âdifferent is betterâ with computers too many times.
6.8 SPC? There is no cheap ammo on the market. Iâd be loading everything I shoot.
300 BLK? Even shorter baseline.
.450 Bushmaster or .458 SOCOM? Even deeper into the well of âtoo unique for my own goodâ calibers.
I regret building a 6.8. As soon as funds allow, Iâm turning it into a 5.56.
If you want opinions, I wouldnât build/buy an AR in anything but 5.56 today. Regardless about what people claim will soon come to market, it hasnât yet. I like the .300AAC idea, but Iâll be the last person to buy it after all these âsoon to be releasedâ parts and ammo variants hit the market at reasonable prices.
Prepare for all the flavored opinions to followâŚ
There are much better options than M855 available. https://www.m4carbine.net/showthread.php?t=19881
I would not focus on another carbine if you donât have at least a few mags loaded up with some of the better performing 5.56 and have taken a couple of carbine classes. You with training and a 5.56 will always be better than you without training and the greatest round ever.
The 6.8 and 300 BLK will both have much better barrier and terminal performance than the 5.56. Most of your training can still be done with a 5.56 carbine and cheaper ammo.
Iâm a big 6.8 fan, and have owned a few big bore ARâs. With that said, and given the purpose of the rifle I would go 5.56. No point in rapidly expending expensive ammo into targets at short/intermediate ranges. Also there are plenty of 5.56 loads out there that will serve you well for defensive use.
If you hunted, the 6.8 or big bores would give you a significant advantage over the 5.56. But for training and (non-work) defensive use the 5.56 is tough to beat.
If youâre looking for an inexpensive round thatâs still decent for SD, you should look at 5.45x39.
I know Spikeâs and S&W make uppers for it. Pretty sure thereâs a few others and lots more options if you can build an upper yourself.
Thereâs plenty of russian surplus ammo still out there. Its usually around 140ish for 1000rds.
If not Iâd just recommend sticking with 5.56 as has already been said.
