I receive frequent questions concerning differences between Hi Powers in these two calibers. My opinions on the merit of forty versus nine millimeter is already posted under the Frequently Asked Question section in "Is the forty-caliber Hi Power a better gun for protection than the 9mm Hi Power?"
Here we'll look at the following questions:
1. Which is more accurate?
2. Which has the better long-term durability?
3. Is there any difference in reliability?
4. How is recoil?
The basic differences in the two pistols is in the width of the slide and the forty-caliber version of the Hi Power is the reason that we now have cast frames on all Hi Powers made by FN. Prior to the release of the forty, all Hi Power frames were forged, but the frame rails reportedly warped or cracked after approximately 2500 rounds. I tend to believe this explanation as Hi Power guru, Wayne Novak, converted some of the FBI HRT 9mm Hi Powers to forty several years ago, but problems in durability were experienced as these had forged frames and the standard weight 9mm slide. Some folks simply hate the cast frames. I have noted zero problems in using them heavily although one downside is that there appear to be some slight dimensional differences from the forged frame upon which aftermarket grips were designed. As the result, some grips don't fit quite as well on the cast frames as on the forged. This may be worse with some production runs of the Hi Power than others. More than one gunsmith I've spoken with has said that both the slides and frames are harder on the cast-frame pistols than the forged. Contrary to what some are saying, the Hi Power slides are not cast. Also, the 32-lb. mainspring did not come as a result of the forty Hi Powers. I don't remember the exact year, but this occurred in the mid to late '70's. Originally, the word was that this was to make sure the pistol reliably fired some makes of foreign ammunition having hard primers. I believe that the change was due to ammunition considerations, but not hard primers as the firing pin springs were made heavier in conjunction with the mainspring power change. During this time, there was much unpleasantness in the Middle East and some 9mm ammo intended for use only in submachine guns was being used in Hi Powers on both sides. Reportedly, the result was rounded locking lugs and cracked barrel cams. The heavier mainspring slows the unlocking process and helps prevent these problems. On today's .40 Hi Power, the barrel has not two, but three locking lugs as opposed to the nine's two in addition to its heavier 20-lb. recoil spring as opposed to the nine's factory standard 17-lb. The additional weight of the forty's slide also slows unlocking and the rearward velocity of the slide.
Most parts are interchangeable, but the most obvious one that is not is the slide release lever. As you know, there is a circular "plate" at the front of the 9mm Hi Power's slide release. The top of this is removed on the forty to allow passage of the thicker slide and there is a relieved area in the slide so that the altered part does not come in contact with the moving slide. A slide stop from a forty Hi Power can be used in a nine millimeter Hi Power, but the reverse is not true unless the top-half of the slide stop "plate" is removed. It also appears that the ejectors are a little thicker on the forty than the nine, at least in earlier guns. I'm not sure if FN has simply started using the same ejector on both pistols or not. While I think either extractor will work in either gun, I could be wrong and have noted that the extractor for the forty has a flat milled on it where the nine's is rounded. Again, I've not tried switching to see for sure if they can be interchanged.
Recoil spring guides, hammers, sears, triggers, and grips are the same for both pistols.
Accuracy: When the .40 S&W was first released and various makers began filling the market with pistols for it numerous complaints concerning a lack of accuracy surfaced. I noted that some of the pistol/load combinations did give less than stellar groups and did so consistently. I no longer see this as an issue. Ammunition makers have had enough time that they've improved and tweaked their products such that the forty is capable of very good accuracy and to levels that most of us cannot shoot to. One major ammunition maker had a premium line of ammunition that did blow cases, but this was limited to a few lot numbers and has long since been corrected. I did personally witness two "kabooms" with their ammo in unaltered handguns, though none were Brownings. Those of you who purchased early forty Hi Powers might remember Browning wanting you to send your gun in for a new barrel. The reason was the ammunition mentioned above. FWIW, the load in question was Federal 180-grain Hydrashok, but this is no longer a problem and hasn't been in years.
In my personal forty, I found that it would group moderately well with some loads, but very well with others. Mine seemed to prefer 180-grain bullets, but an exception was Remington's 165-grain Golden Saber as well as Winchester 155-grain Silvertips. With most 180-grain loads, my forty Hi Power was capable of extremely tight groups. I suspect that this situation has improved as mine was an early gun and the ammo makers had not yet refined their loads to today's levels.
With factory load or handloads that my particular Hi Power liked, it was every bit as accurate as my nine millimeter Hi Powers. From a rest, two-and-one-half inch groups at 25 yards were not uncommon when I did my part shooting from a rest.
Today, I think there is no accuracy issue with either caliber.
Durability: I do not care for forty caliber and wound up selling both my forty Hi Power and a CZ-75B in the same caliber. For that reason I cannot give you first-hand observations on the forty Hi Power's long term durability. I can tell you what I've observed in other folks' forties and what I've been told as well as what I believe.
I estimate that I fired around 3,000 full-power rounds through my Hi Power. Most were handloads, but probably 30% were factory. I noted neither small parts breakage nor undue wear. There was no rounding of the locking lugs.
The Hi Power in forty does appear to be holding up well with continued use as there are few reports of either small parts giving up or catastrophic slide or frame failure. Folks I know who use them report no problems in this regard.
Unlike some gun manufacturers, FN did not just rechamber the 9mm Hi Power and stick in a stronger recoil spring in their forty caliber pistols. They made significant changes in the pistol so that it would withstand the more powerful caliber's use in their gun without beating itself to death.
So long as the forty Hi Power is used with at least a twenty-pound recoil spring and the factory mainspring, I think it will hold up fine for long-term users. I believe that the forty Hi Power will hold up at least as well as the 9mm Hi Power. What I think FN has done in their changes is to make the stresses received to the frame, slide, etc, the equivalent to those found in the nine-millimeter version, which has withstood the test of decades.
Reliability: As most know, Mec-Gar is the maker of the "Browning magazines." This has been the case for quite a few years and has always been so for the forty Hi Power. With my 9mm Hi Powers, either the factory magazine or the Mec-Gar has worked perfectly in quite a few pistols. With my particular forty Hi Power, such was not the case. It showed a strong preference for the factory magazine. Other folks' Hi Powers worked fine with either.
Even though Mec-Gar makes the magazines that come with the Browning guns, the followers are not the same. At least this was the case when I owned my .40 Hi Power. The Browning magazine follower had a "skirt" type arrangement around the bottom of the follower while the Mec-Gar used the traditional "two fingers" protruding or a single oblong post in the middle that the recoil spring attached to. As best as I could observe, the latter two allowed for some tipping forward of the cartridge when the slide hit it from the rear. My pistol would frequently fail to feed on the first shot with a full magazine when using the Mec-Gar magazine, but worked fine with the Browning. I changed the followers in the two magazines and the Browning magazine would cause problems while the Mec-Gar worked perfectly. I've not seen any current Mec-Gar forty caliber followers, so I do not know if this has been changed or not. It should be noted that the Mec-Gar magazines causing me problems in my forty worked perfectly in two other forty Hi Powers with the same ammunition.
Other than that, I've noted no reliability issues with the .40 Hi Power. Extraction and ejection have never been a problem in any .40 Hi Power I've seen or shot personally.
While it may have changed and my main observations are based on but one .40 Hi Power, I did find that the 9mm was more reliable with Mec-Gar magazines. From the Browning-marked ones, there was zero problem with either.
Recoil: As I've stated on more than one occasion, felt recoil is very subjective. What "kicks" to me might not to you! While the forty caliber Hi Power does have more felt recoil than the 9mm, it is not "bad." To me, it is very similar to a nine using hot +P or +P+ ammo, but with more muzzle flip. The rearward "push" seems about like a 230-grain .45 ACP round from a steel 5" gun, but "sharper." If you can handle the great .45 ACP 1911, I don't believe you'll have any problem at all with the Hi Power in .40 S&W.
I'm also asked why I'm not a fan of the forty Hi Power. The reason is that I don't care for the feel of the forty-caliber version. That is subjective and others may very well feel just the opposite. Do not let my feelings on this influence yours; this is a decision you must make, as there is no "right" or "wrong" answer in my opinion.
Were I going to get back into shooting forty-caliber pistols, my first choice would be a Browning.