Barrel length for minimum noise

[cr500]

As I said in a previous thread, I will be building up a 300 Whisper to shoot subsonic loads without a suppressor (illegal to have in my area). I know some people have 16-18 " barrel which make it handy, but I wanted it as quiet as possible with subsonic loads. I was thinking a 24" barrel to get the most velocity out of each grain of powder and to reduce muzzle pressure. Has anyone here compared say a 22" to a 26" barrel to see what the difference in noise is to someones ears a few hundred yards away? I know with my 22RF, when I cut the barrel down from 24" to 20" it got loud all of a sudden.
What length do you think is a good balance between handiness and quietness?

[21contender]

I ordered from SSK, a 21" barrel for my contender in 2005 thinking the longer barrel would reduce muzzel blast and sound of the shot. As you noted a shorter barrel is louder, but what you here is the muzzel blast, I don't know how that relates to shot sound down range. I do know you have to be carefull with load developement. I started with JD Jones recomended loads for subsonic and was suprised how much slower my loads were from his listed vel.
His load for a 220gr. bullet from a 16.5" barrel were 10.3 of 1680 for 1023 fps with extreme spread of 33 fps. Out of my 21" SSK barrel that load was only 764 fps with es. of 107fps. His load with 8.5 of #9 was 1013 fps with es. of 71 fps, mine with 8.6 was 852 fps es. of 36 fps. With that I feel that the bullet is acually slowing down in the longer barrel before exiting. To reach a reasonable speed I ended with 9.7 of #9 for 1070 fps with es. of only 13 fps.
The danger with any subsonic load is a bullet not exiting the barrel. With a longer barrel, that may be more likely. You need to be carefull when you first start to work up loads to check your barrel after each shot. A cronograph helps tell you many things, one is that a bullet went over the screens
Have fun, but be safe.

21c

[cr500]

21Contender. I didnt think that the bulet would realy lose that much velocity with a longer barel. I suppose that these bullets have a long bearing surface. Having to load more powder in to get up to speed probably wont help the noise at all. I might go back to a 20" barrel I suppose. I wonder if an 18" barrel (shortest legal here) would hurt the noise.

[Spook]

If they measure from the muzzle to the bolt face the way atf does here...cut the barrel 18 and have the crown countersunk to 16 or even 14... remember how small the powder charges are in subsonic loads. In my conversations with JD he explained that the charge is spent very quickly and excess barrel only induces drag..... and that will change depending on how dirty the barrel gets.
This round functions just fine out of a 10" Contender if that gives you any clue as to how quickly the powder charge expires.

[21contender]

Personally, I like my 21" barrel length. On the contender frame, the 21" barrel still makes a handy, short rifle. A bolt gun would be longer but I still think getting the muzzel as far from your ears as is reasonable would be a good thing. The countersunk crown sounds interesting. That might act some what like your "bloop tube" from a previous post. The crack you mention in that post comes more from the supersonic bullet than from powder gases.