I have a very slick 9mm Beretta PX4 Compact (Langdon slicked it up).

I want to go to .40 and understand all I need to do it is put on .40 slide, barrel, recoil spring and use 40 mags.

I have all the necessary pieces but they are for a FULL SIZE PX4, not the compact like I have.

The mags stick out a bit (but that's not a problem) and the slide fits on the frame.

There is also a gap (as expected) between the end of the frame and the front of the barrel.

My problem: the slide won't rack. Feels like something is "stuck," and it looks like the end of the recoil spring -- which has a "big" round "button" on the end surrounding the spring while the 9mm recoil spring does not have one is getting in the way.

My question: Will the full size .40 slide, etc. work on the compact frame? Is it me doing something wrong or are the two assemblies simply not compatible?

No the full size slide will not fit on the compact , the recoil spring and cover assembly are totally different . The big question is why convert it at all ???

Thanks for that, and "Ah shucks."

And to answer the question -- someone gave me the .40 pieces and as the frame is what Langdon upgraded I wouldn't have minded carrying around a 40 instead of the 9 in a very nice package, even though performance between the two calibers probably doesn't make any difference in my real world. 9mm self defense rounds are more than sufficient in suburban Seattle as I rarely venture into downtown. (It's one of those "why not" things.)
(Now I gotta find a 40 frame....or figure out if I can use my full-size PX4 45 frame for the 40; which I doubt as the mag wells will be too big on the 45 even if the slide fits.)

Yeah the caliber thing is negligible with good ammo and above all else, proper marksmanship.
The .40 parts wont work on the 45 frame either.
Sorry to be a buzzkill


P.S. Ive carried a Beretta PX4 Compact 9mm for almost the last 8 years . Best advise is to keep them wet with lube and degrease the magazines as they come from the factory with a lightweight orange looking grease that tends to collect stuff and cause issues .
Enjoy shooting !

Just as well, the only tangible benefits to .40 S&W over 9mm are better straight line penetration through hard barriers and greater damage to bones, both of which are only situationally useful attributes, and both are arguably more of a liability in an urban environment.

.40 S&W is better suited for more of a rural environment in which a person is likely to encounter predatory animals with thicker/denser bones and one might have to shoot through brush or something.

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Shooting Comfort is bilateral.

FWIW, I've had Compacts in both .40 & 9, and the top ends don't interchange.

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