A bit of clarification is necessary here for the record. I was the US PM at HK for the MP5/10, from concept to fielding of over 5000 MP5/10's, when the weapon was first developed for the FBI (and the first 5 MP5/40's for the DEA).

The FBI started with the S&W 1076 pistol in back door sole source effort with Smith and Wesson. It later blew up in their face when the pistols did not perform well, even with the reduced subsonic FBI 180 grain JHP loads. Eventually that led to the concept of the .40 S&W cartridge which was later adopted by the FBI in the Glock platform. To save face the S&W 1076’s were quietly phased out as agents who wanted to carry them retired. It was natural progression for the FBI to want a 10mm carbine and sub gun to go along with the 10mm Auto pistol which is how the MP5/10 came to be.

"....the MP5/10 was a finicky weapon, there never was a production MP5/45 for EXACTLY the same reason....and yes I have seen, but did not get to fire a factory MP5/45".

This is gun community myth and is not accurate. The production MP5/10's are extremely reliable weapons on par with those in 9mm. They are still is service within the FBI's Hostage Rescue Team and would not be if they were finicky. Because the way in which the 10mm Auto round had to be "stuffed" into the 9mm MP5 receiver designed for the envelope of the 9mm round, and the barrel extension, bolt head, extractor, and, and, and it does have a few weak spots like the extractor that need to be replaced more often than an MP5 in 9mm. Initially the FBI MP5/10's were issued with the LO Impulse locking pieces and all was well as they were shooting the 180 grain Federal JHP round that was well within the LO impulse range. Problems did occur later when the FBI increased the MV of the round by @ 50 fps without the knowledge of HK and the function of many guns creeped up into the HI impulse range. This was corrected by the use of the available HI impulse piece, especially as the guns break in and as Suds points out below it is wise to start with the HI piece, test 4 or 6 mags for strong function (ejection - 2 to 3 feet from the weapon) and if there are any failures to eject, go with the LO piece. This is not the weapon being finicky. All guns, all mechanical devices for that matter, run within a functional range based on weapon design and the "fuel" fed to it, in the case of small arms ammo. EVERY firearm will perform unreliably when the fuel or ammo is outside the specs used for its design. P7's don't run well on ammo under 100 grains or over 140 as they were designed for 115-124 grain NATO ammo. M4 carbines won't run on lightweight Blue Bolt training ammo without the Blue Bolt installed. Early Benelli M1 S90's did not shoot low brass # 9 reliably in all cases. G3's have over 80 different locking pieces for variables in ammo around the globe. Ford Shelby GT500's won't perform on 85 octane fuel like they do with 93 octane or higher. It is the law of physics in action.

The MP5/10 has to work reliably with ammo ranging from 135 grain to 220 grain at muzzle velocities from under 900 to over 1500 fps! Don't even get me started on muzzle energy and bolt velocity curves - they are immense with the MP5/10. 9mm, .40 S&W and even .45 ACP rounds do not compare. Therefore like in all HK roller-locked weapons, and even more so in the 10mm Auto cartridge, variables in unlocking speed and bolt velocities controlled in part by the angles of the locking piece are necessary. If the customer wants a single weapon that can launch 220 grain subsonic rounds with a suppressor attached and then without the suppressor ding targets at 200 meters with more ME than a 9mm at 25 yards using 175 grain 1475 fps Silvertips the change in the locking piece is required. With this type of HI impulse ammo (like the Norma 170 grain at over 1500 fps) the MP5/10 is the most capable pistol-caliber SMG ever created.

HK never made an MP5 in .45 ACP - that is fact - not even in prototype form. We were asked to do it on no less than two occasions. Problem is the receiver dimensions, and the barrel extension and the bolt head that would have to fit into both simply cannot accommodate the base diameter of the .45 ACP round. That would require a larger receiver so everything that attaches to it has to grow along with it. Just too expensive for the potential sales and the MP5/10 outshines any and all .45 ACP sub guns downrange. This is why the UMP exists today and was first made in .45 ACP. After market MP5's in .45 ACP have a bad reputation for poor performance and broken parts wherein the OD's are cut away to accommodate the larger OD .45 ACP round. HK saw this as an issue and thus refused to ever build an MP5 in .45 ACP.

It was hard enough to get the Germans to build the MP5/10 let alone one in .45 ACP. Certainly a real technical success that unfortunately never went too far beyond the FBI due to the bad rap the 10mm Auto round received as a result of the FBI S&W 1076 debacle, and to some extent the cost and non-standard nature of the 10mm Auto round.

G3Kurz