Have you heard of an exhaust spring modification for the Beretta 390? Can you tell me anything about it?
Yes, I have and yes, I can.
The Beretta 390 vent system is one of, if not the, best. It uses a simple disk covering bleed holes in the front of the barrel’s hanging gas chamber. The disk, or exhaust valve in Beretta-speak, is held under pressure by a two inch exhaust valve spring.
The stock 390 exhaust valve spring functions with an amazing variety of loads, but it can only be optimal within a relatively narrow range of loads. That’s the nature of springs, no fault of Beretta’s. In order to work adequately with light loads, the stock exhaust valve set-up may run a little hot with the heavy shells. By hot, I mean that the bolt speed will be a bit higher than a clay target shooter might want for continuous use over thousands of rounds. High bolt speed, of necessity, translates into more recoil and more parts breakage. The casual shooter or hunter will never notice, but a clay shooter who consistently uses one particular load, might find it to his advantage to optimize the gun for that shell.
It is easy to tell if the bolt is moving faster than it should by how far the empty hull is ejected when the gun cycles. If the hull is being thrown much over ten feet, the bolt is moving rearward faster than it needs to. This means that you and the gun are also taking more recoil and abuse than necessary. High bolt speeds increase connecting rod and hammer strut breakage, the two weakest spots on the 390. An ejected empty consistently tossed about six feed is ideal. That means that your gun is getting enough gas to work reliably, but not enough to beat itself, and you, to death.
Today, you can exactly tune the your 390 to the shell you use most often. Cole Gunsmithing makes a set of five different color-coded Beretta 390 exhaust valve springs. They measure 26, 31, 36, 45, and 51 lbs at 25% deflection. The stock spring that came with the gun is around 41 pounds.
By experimenting with the different springs, you can literally match your 390’s gas system to a particular shell to minimize recoil and parts breakage while maximizing reliability. Just follow the complete instructions included. Try out the various springs until you find the one that ejects the hull of your favorite shell about six feet. If the hull is being thrown too far, you use a lighter spring to vent away more of the gas. If the hull doesn?t go far enough, use a heavier spring to keep more of the gas to operate the action. Nothing simpler.
I tried the Cole spring set out with both light and heavy loads. With the heavy spring in place, my light 1150 fps 7/8 oz reloads ejected smartly instead of just dripping out the way they sometimes did with the original spring. With heavy 3-1/4 dram 1-1/4 oz FITASC loads, by using one of Cole’s soft springs to vent much of the gas, I was able to slow the action down and actually cut a little recoil, while still retaining reliable ejection. If you feed your gun a steady diet of rhino rollers, it will definitely last longer with the right spring in place-and so will you.
If you like to tinker with your 390 (and who doesn’t), it is definitely worth a try.
The Technoid writing for Shotgun Report LLC