Bore And Choke Size

Dear Mr. Buck,

I am an aspiring Jr.. technoid in training.

My question: I recently bought a A390 Sport Trap, 32 in barrel, ported. The inside barrel dimension is .718 and the” full” choke measures .688 (30 points of constriction). The manual lists the bore as .725. It appears to pattern as “Full” on the pattern board. I have not shot it enough to tell if it is going to work for me yet.

Does .718″ seem too small? Is this in the normal range for a Beretta A390?

Concerning cleaning the gas piston: I put mine in the dishwasher with a load of dishes. It came out very clean. (Explaining to my wife exactly what I was doing, took longer than the actual cleaning process.)


Mark Technoid in Training

Dear Mark,

You don’t mention whether your 390 barrel is a solid choke or screw-in Mobil choke. I will assume the later as that’s the way most come.

I’ve never miked a Beretta 390 barrel as tight as .718″. The general range that I have seen is .722″ to .724″. I don’t think that it matters a lot if the gun patterns properly, but it is snug. By the same token, .688″ seems about right for a Beretta “Full” choke. most of the ones that I have seen have been have been around there. Of course, that gives you about .035″ or so in a “normal” Beretta .723″ barrel. .035″ constriction is sort of, more or less, vaguely, industry “standard” full choke.

Here’s something to try: Briley will sell you chokes of any exact measurement you want. I couldn’t get much better than 72% with my favorite shells in my barrel/Full choke combination of .035″, so I had them cut a choke for me which gave me .040″. That got me up over 77%, with occasional patterns even higher. When you get around .040″ constriction, many guns start to give diminishing returns. It really depends on the other interior factors of the barrel. Pure choke constriction is only part of it. It can also become very pellet size sensitive. That’s why most makers like to stick with .035″ at the top end as just about everything works pretty well in that.

The bottom line to it all, though, is not any particular measurement. It is performance. Measurements may give an indication of performance, but only pattern testing will tell you for sure. Yes, I know it’s easier to mike a barrel and choke than it is to pace off 40 yards and run three patterns, but that’s the only way to know for sure. Other than that, you are just guessing. You say you have patterned it, so you know what you are getting. If it isn’t tight enough, order up a tighter choke. I wouldn’t worry about the .718″ bore if it delivers the patterns.

The only place that the tight bore would bother me would be if I were using the gun for sporting clays or other games where you have to change chokes a lot. You would have to re-index all the other chokes in your kit, remembering that your bore was .005″ tighter than nominal. That’s a pain.

I love your idea of cleaning the gas piston in the dish washer, though I have never tried it. I do know that some people boil it in hot, soapy water and report good results. It reminds me of when I used to clean my hulls in the clothes washer and dryer. Talk about getting flak from the hausfrau!

Best regards,

Bruce Buck

The Technoid writing for Shotgun Report, LLC

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1 Response to Bore And Choke Size

  1. blackwellopt says:

    Briley is in the choke manipulation business, ought to know since I live here in Houston. Will witness the calipering. I have them stamp the ID of the choke, constriction, label them, & stored on iphone contacts. Of course I can do the math. Referring to my 2002 Edition Of Black’s Wing & Clay, a full choke has a 0.040″ constriction vs today’s 0.030″ constrictionas per Briley. Gas piston put it in mason jar with low odor mineral spirits & soak for several days. Same program for chokes. I do not leave my chokes in that MX 8.


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