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The Annoyed Man wrote:Quick question..... Do any of the changes affect whether holsters for gen 3 pistols will fit the gen 4 guns?

Should be no effect on holster fit.

Upper receiver & recoil spring fit is biggest change ... i.e. Items like dedicated .22LR uppers (Tactical Solutions, Advanced Armament) are NOT interchangeable between Gen. 3 & 4. This is because the new double recoil spring requires more internal space in the slide. & frame .... thus , you can't mount Gen 3 fit slide to Gen 4 frame etc ... but the external dimensions important to holster fit are unchanged

All of the above benefits of Gen 4 PLUS the double recoil spring does reduce felt recoil - noticeably if you fire a Gen 3 & Gen 4 side-by-side.

On the plus side for Gen 3, the older Gen 3s still have the better/more durable Austrian-made Tenifer slide and barrel treatment - superior in most knowledgeable opinions to the American-made version. Haven't owned my Gen 4 Glock long enough to know first-hand, but my older Gen 3 Glocks never saw a spec of rust while my USA-made S&W M&Ps will rust if not cared for. Something about the Austrian Tenifer process is illegal in US manufacturing (EPA rules), but I'm fuzzy in the details.