300 BLK Buffer

[SC Mike]

I just put together a 300 BLK 16'' carbine upper w/ a carbine length gas tube. The barrel has a .125 gas port and my gas block matches. I used a lower with a conventional A2 fixed buttstock to test fire. It shot great except that it does not hold the bolt open on the last round in the mag. Then I used a lower w/ a M4 style carbine buffer/ spring. Now the bolt stays open on the last round. After doing some homework I found that because of the buffer weight and the length of the barrel, when using a carbine upper, a carbine buffer and spring should be used with the appropriate stock assembly. Here's my problem- I prefer the conventional fixed stock. It fits me and I think that it looks better. The spring lengths aren't close so that leaves the buffer. The carbine buffer will fit in the A2 buffer tube using the rifle buffer spring (tried it). But I am wary of shooting it because of the possibility of damage. Is this a viable solution? Can some of the AR experts/ experimenters give me some feedback. Worst comes to worst and I'll just use a M4 style stock. Don't want to start cutting springs, etc. Responses appreciated.