Hey all I'm working on 2 builds at the same time and for the first few months they are going to have to share a lower.
1 is a 16" 6.8, the other is a 16" .300AAC.
The 6.8 will be a mid length Gas system (ARP), and the .300 will be a carbine gas system (no sub sonics being used).

I see a lot of people running an H2 buffer with a Springco Red spring in their 6.8 rigs...My question is, will this still be a good setup for the .300?? Also, would the Spikes ST-T2 Buffer be ok to use in place of the H2 since I already have one? I think the spikes is a little lighter 4.05oz compared to 4.7oz in an H2 if my memory is correct.

Personally, I would just try both uppers with what you have on hand. If they both work fine with what you have there is no need to buy another buffer.

I was under the same predicament when I built my AR pistol after reading so many problems with buffers, but when I went and shot it with the one that came with the buffer tube it ran flawlessly.

surprisingly the spikes ST-2 is the one I have on hand lol....I had it in my .223 rig before switching to a rifle buffer....the only thing I need to buy is a spring then

I have a 5.56 carbine upper and a 6.8 rifle upper and use the same carbine buffer with both. I haven't had any problems with either. The main thing to remember is use a carbine buffer with an adjustable carbine stock and rifle buffer with an A1/A2 fixed stock.

yea def. I only switched my .223 to a rifle buffer because I went to the full length Ace stock