^^^

Depends on how your upper is gassed...

Any buffer is okay in any length if it:

This!  If it cycles then yes, if not.....

The above is true, but it's highly probable that you will need H2 or H3.

I use an H2 with my Ultra Match 7.5" 5.56

Dave N

FPNI .....      depends on the gas port size.   You want the heaviest buffer that will still allow the bolt carrier to fully cycle. A buffer that is light will let the action cycle, it's just going to beat the action up, causing premature wear. A buffer that is too heavy is not going to allow the carrier to cycle fully.  I guess you don't have to, but using the buffer that is "just" light enough to have everything cycling properly, but without excessive bolt travel speed, is the one your firearm wants.
   I see lots of people make comments like "I run the std. carbine buffer in my 10" (enter any particular brand) carbine and it works fine".   What they don't know is that the manufacturer already put a large gas port in because the man. wants that carbine to run no matter what ammo is used, how dirty it is, etc.   So........YEAH........it runs, but it would be a much smoother running action if the bolt carrier wasn't being slammed at such a highly overgassed/underweighted buffer condition.

+1

I never found a car buffer that wouldn't work in a 14.5 car gas system

And this is exactly what I am talking about. Sure, you can run a standard weight buffer in any situation and it should work. The thing is, the firearm might be hammering itself away needlessly.
So.........again.....if you run the heaviest buffer that will still let the action cycle reliably you will have a smoother, lighter recoiling firearm. Is that important to most people? No, but if you want the smoothest, lightest recoiling firearm, to get back on target faster for multiple or follow-up shots then "lightest" isn't necessarily the way to go.

thanks everyone for the help. im not positive about the gas. its a 7.5 noveske diplomat

I ran a H3 in my Diplomat with no issues across a wide range of ammo.

Solid advice..I typically bring standard to H3's to the range when I test out new builds.

This, the only way you're going to know for sure is to try it.

I believe Noveske used an H2 in their diplomat builds so I also put an H2 in my 7.5 diplomat barreled pistol and it has run flawless from the get go.