What person or company decided 34” should be the proper length of most trap barrels? I get that we see 32’s but not too many. Why not a metric length? Why not 33.75?
All of Dennis DeVault's guns were 33". remember the old two barrel sets 32" for singles, 34" for caps Honestly, if you're past a certain age and have lost weight & muscle mass the lighter & shorter may be better. Its a shame now that we have the resources & time to shoot our bodies are no longer up to the game. If I knew I would last long enough to shoot in well I would get a new TM9X,(32") with a Russ Diguesaldo fitted stock
It's an interesting question, but I think you'll learn that 34" barrels break targets, best, especially when the same gun is being used to shoot singles and long yardage caps. Doubles, on the other hand is a different game, and there's room to play; but it all comes downuh to expense. I mean, how many combo barrel sets can you afford to buy? Also consider the 1 1/8oz #7 1/2 shotshell which is nearly perfect for breaking targets at ~40yds. Stated simply: it's what works best for most competitive shooters.
34" barrels date back to 1853 from my research and I have no doubt they go even further back in time. In 1906, Winchester assisted a notable shooter with experimentation of guns with 32" and 34" barrels both using the same loads. Over 10 shots the 32" barrel averaged 966fps and the 34" 969fps. It was done at Winchester's range and supervised by one of their ballistic experts. Here's the shooter's opinion on 32" vs 34" barrels.
Maybe thats why an auto-loader works well, considering the long receiver , you have a real long sight plane, if that is important to you