Shim Adjustment


I am a beginner with clays and 5 stand and own a 391 SPORTING Model. I am not sure how to interpret the shims and cast on/off…. I am of average height (5-9) and the shims from the factory are set at c (60 dx) I think. It appears from patterning the gun this past weekend that I am at about 60/40 maybe 55/45. What do the shims mean from a-d and 50-65. and when someone says raises comb height what exactly does that mean – More bend in the stock? What would you guess to be the right fit for me? I know that boils down to person to person but would you mess with the factory set of c 60dx?


Dear Chris,

Here’s the real deal on any kind of stock adjustment: stock adjustment has absolutely NO effect on where your gun places its pattern IF (note big “if”) you maintain the same sight picture.

Example- let’s say that your stock is real low for you. You have to sort of pick your head up a bit to look flat down the rib. When you do look flat down the rib, the gun shoots 50/50. Now you raise the stock a good bit. But, by scrunching your face down hard on the stock, you can still look flat down the rib. The gun will still shoot 50/50 for you. Although you have raised the stock, by manipulating your head you have maintained the same sight picture and thus the same point of impact.

The ONLY way that moving a stock up, down or sideways will affect the point of impact is if you change your sight picture on the rib. If you see more rib, your gun will shoot higher, less rib is lower. Moving the stock will change your rib sight picture if you maintain a constant cheek pressure on the stock and just forget about the rib. Shoot off the front of the gun, not the rib.

The difference between moving a stock and moving the eye in relation to the rib seems so basic that I am often surprised that some people don’t consider it. Moving the stock moves your head in relation to the rib, and thus the point of impact, only when you maintain the same cheek pressure. It’s easy to check this if you mount the gun with your eyes closed. That way you can concentrate on keeping the cheek pressure the same and won’t be distracted by the amount of rib you see.

So now that’s out of the way. The basic rule of thumb is that your point of impact moves in the same direction that you move your stock. If you want to make your gun shoot higher, you make the stock higher. If you want to move the pattern to the right, move the stock to the right. It’s like the rear sight on a rifle.

On Beretta gas guns, the stock shim sets (front plastic ring and rear steel washer) indicate height and cast. The number indicates the height in millimeters from the heel (top of butt) to an imaginary line parallel to the rib. This way a drop of 50 mm results in a considerably higher stock than a drop of 65 mm. The bigger the number, the lower the stock and the lower your point of impact should be. If you shoot 60/40 with a 50 mm, you might shoot 55/45 with a 55 mm and 50/50 with a 60 mm. Something like that, sort of.

The SX and DX refer to cast off. S is for “sinister” or “left”. In fact, with Berettas this is no cast at all. It’s a neutral stock. D is for “dexter” or “right”. With the D shim in place the stock is shifted to the right and point of impact will move to the right. It’s supposed to be able to accommodate a right handed shooter and permit him to shoot with his head up, rather than turned slightly into the stock. I’m a right handed shooter and feel most comfortable with the SX shim because I turn my head slightly into the stock and thus don’t like cast off.

I don’t have any idea what the right settings for you might be. If you can raise the gun quickly to your face and immediately fire to an aiming mark and hit it, the gun is about right. If you are a bit high, say 55/45, that’s OK. 60/40 is too much for sporting, but many people like it for trap. Personally, I like my sporting guns set up 50/50 because you get just as many dropping targets as you do rising ones. No matter what the game, you want to be correct on the left and right measurements. You don’t want your gun shooting to one side or the other no matter what the game.

That’s the nice thing about Beretta’s shim set. You can fuss around until you find what you want- or just plain give up, whichever comes first.

Best regards,

Bruce Buck
The Technoid writing for Shotgun Report, LLC
(Often in error. Never in doubt.)

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1 Response to Shim Adjustment

  1. says:

    Finally an understandable, in depth answer to a sought after question.
    Thank you


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