Please advise on shortening barrel 20 to 14"

Bokes, 

There are a few factors involved in this. What caliber, velocity, weight of pellet, kind of gun, type of barrel (smooth twist, straight rifled, rifled with choke). If you reduce hammer spring tension, you will get the valve to close faster. This results in lower velocity. So, with that, you would have to open the transfer port (If you are able to, some guns cannot be adjusted in that manner), or, you have to make the valve spring weaker. In my opinion, you are not going to gain a lot. If you look at an average Field Target 12 ft. Lb. rig, the barrels are usually around the 20-23 inch range, and, they are pretty efficient. If you shorten the barrel that much, without knowing a lot of the info I mentioned above, you are going to loose efficiency, and power. Bottom line is, you can't get something for nothing, and in my opinion, shortening a barrel that much will hurt you, more than it helps you. The longer the barrel (within reason) the more efficient, consistent, and stable it would be. 
I'd leave that barrel alone, it's a decent length for lower power/efficiency. Play with hammer spring, stroke, valve spring pressure, and transfer port to get the velocity to where you want it. You'll thank yourself for doing so.
Any questions, don't hesitate to contact me, and give me a shout.

Tom Holland [email protected] 
Assistant Match Director 
EasternSuffolk Competitive Airgunners Association (ESCAA)Long Island NY escaaclub.com 

Again, just so I have a clearer picture, what gun and setup do you have? Most, 12 ft. Lb. guns, in .177, run a minimum 17 inch barrel to be at the lower rifle efficiency, with tweaks to the valve/transfer port/hammer spring. Longer barrels will give better efficiency up to around 24" or so, not longer than that. I have made shorter barrels 12 ft. Lbs, and without shroud help, do bark at best. If you want a 12 ft lb. Gun, I can steer you in the right direction, depending on your rig. If you have to have a short 14" barrel rig, at 12 ft lbs, its not going to be an effective air saver. Generally, the longer the barrel, the more efficient the system is. The shorter, the less efficient. At a 20 inch barrel, not knowing anything else, I would not shorten it. Mess with all the other stuff, you always can put it back the way it was. Once you cut a barrel down, you are stuck with it, unless you want to buy a new barrel. I would try to adjust your rig to get 12 ft. Lbs, as is, and see what your shot count looks like. Again, in my opinion you will gain nothing you are looking for by reducing the barrel by 6 inches. You'll have to put MORE air through a barrel that short, to get to 12 ft. Lbs, thus using way more air, being way less efficient, and a lot louder.
I'm not trying to discourage you in your project, but, as i have mentioned above, you can't get something for nothing, or little. This air delivery thing has a mind all of its own. 
Again, hope I can help,

Tom Holland [email protected] 
Assistant Match Director 
Eastern Suffolk Competitive Airgunners Association (ESCAA)Long Island NY escaaclub.com 

Answer depends on whether it is regulated or not. If regged YES, you can just turn down the hammer spring, and that will lower the power without wasting air. Only thing s you will have to stop shooting when it comes off the reg as your shots will likely get faster. The "perfect" way to do it is to lower the reg pressure till you get about 15fps more then you want. Then back off on the hammer spring till you get the velocity you want.

If not regged then you will likely have to lower your fill pressure and also back off on your hammer spring. Takes a bit more time to do then a regged gun, IMHO

Bokes,

I think you answered your own question. I now have a better understanding of the rig you are trying to modify.
. If you reduce the barrel to the length that you want, you will be approaching the length of the Ranchero carbine anyway. It'll be disappointing for you to shoot, because any barrel that is that short at 12 ft lbs. will have a bark to it, no matter what you do with all adjustments, and you'll need an LDC anyway, and as you said, you are trying avoid.
Summing everything up, if you shorten the barrel, you'll loose velocity, which is what you are looking for. It will get much louder, and, even at 12 ft. Lbs., will have a bark to it, that you will not be able to adjust out, and still retain your desired velocity.
I just don't want you to do something that you might regret down the line.

Good Luck

Tom Holland 
[email protected] 
Assistant Match Director 
Eastern Suffolk Competitive Airgunners Association (ESCAA)Long Island NY escaaclub.com 

Bokes, 

I think that if you can, and had all of the above adjustments, including the regulator, I would think that you would be able to adjust for it. Whether or not you are going to waste air or not, would have to be seen.
Keep in mind, a shorter barrel is going to use more air than a longer barrel of the same power.
I had my Marauder adjusted in such a way, it shot awesome. The drawback of that, I was wasting air. Only about 65 shots at 12 ft.lbs, when everyone agrees I should be getting well over 120. I eventually tuned it where I'm getting 130 shots a fill, but the awesome accuracy isn't there anymore. Don't get me wrong, it's still accurate enough to compete in WFTF PCP class, but not like it was. In the long run, I've learned the hard way, you can't have everything you want.
Hope this helps, good luck, keep us posted,

Tom Holland 
[email protected] 
Assistant Match Director 
Eastern Suffolk Competitive Airgunners Association (ESCAA)Long Island NY escaaclub.com 

"bokes"thanks Tom,
My purpose for this rifle is not 1-hole accuracy, it's quick handling & decent accuracy.
I think there'a big difference between using more air & wasting air. I'm ok using more air, but I don't want to waste it. Clearly if the pellet has left the barrel while the valve is still open, then that's wasted. This is what I want to avoid.
My understanding is when a barrel is shortened, the valve needs to close quicker to prevent this. Correct? Anyone else do this?

Click to expand...

Yes - I have done it at least twice. And you are correct about the valve. Basically you raise the reg pressure so that you get a "quick" burst of high pressure as opposed to a longer burst of lower pressure air. I have a QB79 that I took 6" off the barrel. I put a 13ci bottle on the gun and cut the barrel so that the LDC is right after the bottle. Makes a nice short carbine. I'm away from home right now so can't take a pic. I have the reg set at 1400psi. The gun is shooting between 15 and 17 FPE depending on the pellet I use. This is in .177. It does not waste air and has a very quite "short" report with the attached LDC.

You would be amazed what you can get out of a short barrel. I have a BSA Ultra that has a barrel under 12". It is shooting very efficient at almost 30FPE.

Below is another QB79 that I built last year. It's just about the same length as the one I have now. (With the LDC it is still shorter then a stock QB with no LDC.
This one I regged at 1000psi and tuned it to shoot 12fpe.

I shortened the bbl. on my Maximus .22 by 4 1/2". I was bumping it with the LDC on it on everything in the woods. LOL!

It is now short, handy, and just as accurate as it was before. It has a 6" Rocker! Carbon fiber LDC on it, with Huma reg, and Mrod gen .1 trigger, BNM Multi shot breach. 
Knife