Bushmaster ar15 5.56 to 6.8 conversion??

Question

i have a bushmaster ar15 m4a1 that fires the 5.56x45mm (.223rem). i know that bushmaster sells a upper toconvert it to 6.8mm and i can keep the same lower. Is there a big diffrence from the 5.56 to the 6.8? and is ammo cheaper and where can i find it. i tried looking but cant find it anywhere. i also read somewhere that the 6.8 is a .270. can i use .270 shells in it, are they the same? will my accuracy suffer with this round? oh and do i need to buy 6.8mm mags or can i use me 5.56mm magizines?

is it worth just getting the upper or put in alittle more money and get a whole other gun..cuz who doesnt need another AR!!!!

thanks in advance

Answer 1

Here's the idea behind the 6.8 SPC, and from there you can figure out the rest.

It's been known for a century or better, at least by some experts, that the ideal compromise for a combat cartridge would be a 7mm bullet of about 120-130 grains at around 2500 fps. The British were working on development of such a round when WW I intervened, and had another well into development when the US forced the 7.62x51 down NATO's throat. Additionally, it would be nice if the case were as small as possible to minimize the size of the action and maximize the number of rounds a soldier can carry. Add to that the huge number of M-16 rifles and M-4 carbines in the inventory, and the practical idea that the US isn't changing rifles anytime soon, and it was sensible to get as close to that ideal as could be fit into the AR-15 action. Well, the 7mm wouldn't quite work, but .277" bullets were close enough, with a 115 grain bullet (comparing this with the usual 130 to 150 grain bullets of the 270 Winchester). The case of the 5.56x45 is about as long as the AR-15 platform can handle, but to get the needed amount of propellant into the case, one was needed with a larger base diameter than the .378" of the 7mm TCU (a necked-up 223 case), and smaller than the .473" of the 30-06/270 Win/308 Win family of cases. An obsolete Remington autoloader case fit the bill and was resurrected for the purpose. The SPC round is also known as 6.8x43, and a metric designation of 270 Win would be 6.8x63. I think you can figure it out from there.

Answer 2

The 6.8 SPC has a bigger heavier bullet thus better down range performance.  The 6.8 SPC will be much more expensive than the 5.56N.  Only a few two or three companies make it at the present time.  .270 Win and .270 Wby are to long to be loaded through a 5.56N AR platform.