3-1/2″ Chamber Velocity Loss

Dear Technoid

By the way,you didn’t answer my question about the 3-1/2 inch shells. Is it too much shot through a 12 gauge to pattern correctly?

Can’t wait to hear the scoop!


Dear Rich,

As to the 3-1/2″ chambers when using 2-3/4″ shells, I don’t know about patterning as that varies with the shell, but there may be a velocity loss. Several of SR’s readers have felt that there was. Here’s a comment from Randy, one of SR’s loyal readers and holder of the coveted Junior Technoid Order of the Palm (3rd class):

“Technoid Info Dept.: a few recent Chrono sessions revealed a consistent whopping 100+++ fps drop in speed using 3-1/2 12 ga. chambered guns vs. 2-3/4 and identical loads. Specifically, Benelli autos. An amazing drop off in performance, more than one gun, and yet another reason to stay away from 3-1/2 chambers unless you are REALLY going to use them.”

I haven’t done a nose-to-nose test the way Randy did, but I sort of suspected that there was a problem. I have noticed off-sounding shells when using promotional loads on a cool day when using a gun with “extra long” forcing cones. Not all wad skirts are happy sealing an .800″ or thereabouts bore under all ambient temperatures. Heck, some of the promotional wads I have tried produced bloopers in standard barrels if you let the shell get cold enough by leaving it in your trunk overnight in the winter. The more open the chamber/cones/bore, the worse the problem gets.

On the other hand, Stan Baker (the well respected Seattle gunsmith and columnist), builds a “Baker Big Bore” barrel conversion. These barrels are basically super-overbores, measuring .800″ from rim cut to choke. That’s like having a 30″ 12 gauge chamber. Baker claims an INCREASE in velocity of up to 50 fps due to the reduced friction . I never had the chance to chronograph the gun, but I did notice when I shot a “Big Bore” conversion on a Perazzi that it had a “hollow” sound on cool damp days.

It is obvious that modest overboring doesn’t hurt anything when you use a good shell and equally obvious that there is a point at which a bore become so large that it does. This will vary from shell to shell and with the weather (cold hardens the wad and may keep the skirt from obturating properly).

All of the above comments involve velocity, not patterning which you ask about, but it gave me the chance to spread a little ink in the ether. Pattern quality is somewhat linked to velocity, but not irrevocably so. Generally and with all things being equal, the faster you push the shot, the less consistent and more open the pattern. Carried to its logical conclusion, the consistent loss of 100 fps due to the long chamber could TIGHTEN your pattern very, very slightly- unless of course gas blow-by became an issue. Then it would loosen your pattern. Oh, heck I don’t know how many angels can dance on the head of this pin. Take the darn gun out and pattern it! Then you’ll know and I won’t sound so ignorant and confused.

Best regards,

Bruce Buck
The Technoid writing for Shotgun Report, LLC

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