Powder burn is complete by 13-14" barrel length for standard velocity .22 in most cases. High velocity "might" use a couple more inches of barrel length for peak MV. Anschutz and others offer 14" barrels in Europe on their rifles, usually for suppressed use on squirrels (NFA is seen as ridiculous over there). See this link for general info:
http://ballisticsbytheinch.com/22.html
You will be quieter with a longer barrel as powder burn has completed and now the propellent gases are just filling a longer bore and slowing down. I do hear a difference between 16", 20" and my 28". Not enough difference to justify 4" or more of barrel length to me though. Just put a suppressor on it and it wont be a problem for anybody.
Getting a random supersonic crack is the ammo maker's fault as ALL .22 ammo has crappy muzzle velocity consistency, especially the cheaper ones. I've seen Extreme spreads of 68 fps in just one 50 rd box of Aguila SV ammo before. The ES starts dropping as the price goes up, but it's pretty rare to see it drop to single digits, even on match grade. I've even had boxes of TENEX and EXACT give dissappointing MV results, so I've pretty much gone to SK Rifle Match red box and LAPUA Center-X as the best match compromise of price vs consistent MV. (both are made by Lapua, different machines though). Honestly, after seeing videos of how .22 is made, I'm amazed that it is as consistent as it is.
Match rifles use long barrels for different reasons: longer sight radius, vibration tuning and tuners, and hand vibration(hand shake) damping. Most of my competition .22 rifles use 16-18" barrels, I found no advantage with longer barrels, especially in PRS type matches. The one exception to this is the Bench-rest gun, and that is for barrel tuning and vibration, but it shoots at only one fixed known yardage and weighs over 14 lbs anyway.
