As I look at the NRA Marksmanship Program catalog, I notice that pretty much things seem to jump from .22LR straight to full-length 20" Service Rifle. Are there any disciplines other than 3-Gun with formal competition in the 14-16"-barrel range where the GF and I can put my M4 and her C8 in play, or am I gonna have to bite the bullet and build us a pair of M16-length uppers?
And you could always run your 14+ inch guns against the 20 inch guns. You would be at an disadvantage but if your just having fun, have fun.
Or you could buy one of those badass FN MILITARY CLASSICS 20 inch guns. Good excuse to buy one, your welcome.
PB
Write a check, where’s the fun in that? For me the challenge is as much in the craftsmanship as the competition, if not more… 
Look up your local ranges, even the private ones. Most people overlook the ‘members only’ places that probably allow the dirty public in for competitions.
M4geries/SPRs are authorized in National Match and CMP Games (Modern Military Rifle) competition.
We hold reduced distance High Power Rifle Matches at our gun club and any rifle or carbine that groups well at 100 yards is competitive. I’ve seen people shoot decent scores with a carbine. With the introduction of optics in the High Power game, sight radius is a non-issue.
Scoring 10’s and X’s at 600 yards with a carbine will be a little tougher than with a rifle length barrel. At distances of 300 yards or closer a good carbine barrel with good ammunition is good to go.
Two gun matches are gaining in popularity. Look up the Desert Brutality matches on you-tube…