barnbwt
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- Aug 14, 2011
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I got the first test shots run through my conversion project today, and things went perfectly! This is a brief synopsis since there is an excellent summary out there by lifesizepotato on Youtube already, and the various mods are pretty self-explanatory. His pictures/video are better than I can produce, as is his narrator's voice
This chambering-conversion (caliber is the same, whatever) is rather unique and appealing because;
1) It seems to work very reliably (i.e. everyone I've heard of doing this has had good results)
2) Requires minimal rework (more on that later)
3) Is based upon a high quality pistol, and does not seem to overload it's strength
4) Doesn't (necessarily) require much money to complete compared to others
5) Uses a consistently cool, but not terribly obscure or expensive cartridge with few drawbacks
6) There's also the historical connection between HK of this era and the 10mm Auto because of the MP5/10 contract run they did. I'm not sure why HK never offered this gun in 10mm since it's so easy & they had LEO contacts using the subguns for years, but it's probably because the FBI had already punked out on their 10mm commitment by the time the idea occurred to them.
The modifications to the gun consist of the following:
1) 40 S&W USP40 full-size barrel rechambered to the longer 10mm Auto
2) Front of slide ejection port relieved (optional; only required to properly eject unfired rounds)
3) Plastic HK magazine replaced with a steel STI/Para 14-round double-stack 1911 magazine
4) Stock HK recoil spring replaced with a stiffer version (Wolff is a noted source)
Now, historically people have replaced the HK barrel with an aftermarket EFK Firedragon/etc in order to do the chamber reaming. HK barrels are surface-nitrided, which makes their surface slightly softer than silicon-carbide. This, as you may imagine, is hell on high-speed-steel chamber reamers, so they cannot be used at all in any fashion. The aftermarket barrels are made of the more garden-variety 41-series chrome-moly alloy steel at a reasonable through-hardness, so they can be freely cut by chamber reamers. Tragically, EFK has ceased production of all full-size USP40 barrels at this time, and has no plans to resume. So despite the popularity & obvious advantage of the USP10 conversion, most builders are stalled out these days.
But, where there's a will, there's a way, and practically every type or hardness of steel in existence can be cut by tungsten carbide.
The first step in rechambering the barrel as I did is giving up on getting a new or used EFK barrel at any price, as well as shooting the USP40 in its original chambering. It shoots well, but is as dull as dishwater and rather big for what it is, therefore useless to me for range-play as well as carry. Now that we're willing to ruin the barrel in pursuit of success, we can proceed;
The task is to bore, with a single-point carbide cutter, an extension of the existing chamber along with a leade, such that the 10mm can fit properly. This is way more fiddly and tedious than even careful chamber reaming, with a lot of potential for barrel-ruining screw ups, which is why reamers are preferred in the first place. So builder beware. Since each precision surface is being created individually, there is much more measurement, math, measurement, double checking, and measurement involved. Mostly because you can't see anything that's going on inside the gun barrel while you're doing it (so you have to function as an 'instrument rated' machinist or garage tinkerer)
The good news, is that the cheap carbide boring bar I used (nothing special, basically a round rod with the necessary reliefs & square cutting edge ground, for like 10$ on ebay) bit into the metal like it was nothing at the highest speed my lathe could turn, at cut depths as shallow as .002", and left a very smooth surface. This is the ingredient that's been the bane of so many other conversion attempts, now that aftermarket barrels are gone (lifesizepotato mentions sending his HK barrel to three different smiths for reaming, fruitlessly, before turning to EFK)
The bad news is it is really easy to screw up, unless you've got a very good indicator setup and a thorough process to keep track of everything. In my case, I'm a hackneyed garage tinkerer, so I have to rely on fit-check and simplification of the job more than someone better equipped. Rather than match the very subtle case taper of the 10mm vs 40 S&W chamber, I just cut a straight-walled extension to the original, gradually widening its diameter until an unfired case could drop in/out freely without catching. Rather than cutting the proper SAAMI-profile leade that has a straight then tapered section, I cut a single taper between the start & end points (a tiny bit tighter for certain bullet profiles). My chamber actually ended up a tiny bit deeper than it should be, but it doesn't appear to effect function outside of very unique/odd circumstances (primed, empty cases jump the extractor claw when fired & must be tapped out with a bore rod; live rounds have no such issues or irregularities). In summary, gotta be careful.
The best news is the gun works great so far and is a hoot to shoot. Very much more fun with this slightly more powerful cartridge (which can obviously still get a bit more potent than FMJ practice rounds if needed). The muzzle report, recoil, and authority of 10mm on steel plate seems much more appropriate for the size and weight of the gun. I was single-feeding rounds from my Para P14 mag since I was too impatient to test the barrel to complete the mag mod, but feeding worked just fine. Extraction & ejection were...very aggressive with the standard recoil assembly; MP5 would be comparable. Gonna definitely replace that with a beefier unit so the ejector/extractor aren't working so hard.
TCB
This chambering-conversion (caliber is the same, whatever) is rather unique and appealing because;
1) It seems to work very reliably (i.e. everyone I've heard of doing this has had good results)
2) Requires minimal rework (more on that later)
3) Is based upon a high quality pistol, and does not seem to overload it's strength
4) Doesn't (necessarily) require much money to complete compared to others
5) Uses a consistently cool, but not terribly obscure or expensive cartridge with few drawbacks
6) There's also the historical connection between HK of this era and the 10mm Auto because of the MP5/10 contract run they did. I'm not sure why HK never offered this gun in 10mm since it's so easy & they had LEO contacts using the subguns for years, but it's probably because the FBI had already punked out on their 10mm commitment by the time the idea occurred to them.
The modifications to the gun consist of the following:
1) 40 S&W USP40 full-size barrel rechambered to the longer 10mm Auto
2) Front of slide ejection port relieved (optional; only required to properly eject unfired rounds)
3) Plastic HK magazine replaced with a steel STI/Para 14-round double-stack 1911 magazine
4) Stock HK recoil spring replaced with a stiffer version (Wolff is a noted source)
Now, historically people have replaced the HK barrel with an aftermarket EFK Firedragon/etc in order to do the chamber reaming. HK barrels are surface-nitrided, which makes their surface slightly softer than silicon-carbide. This, as you may imagine, is hell on high-speed-steel chamber reamers, so they cannot be used at all in any fashion. The aftermarket barrels are made of the more garden-variety 41-series chrome-moly alloy steel at a reasonable through-hardness, so they can be freely cut by chamber reamers. Tragically, EFK has ceased production of all full-size USP40 barrels at this time, and has no plans to resume. So despite the popularity & obvious advantage of the USP10 conversion, most builders are stalled out these days.
But, where there's a will, there's a way, and practically every type or hardness of steel in existence can be cut by tungsten carbide.
The first step in rechambering the barrel as I did is giving up on getting a new or used EFK barrel at any price, as well as shooting the USP40 in its original chambering. It shoots well, but is as dull as dishwater and rather big for what it is, therefore useless to me for range-play as well as carry. Now that we're willing to ruin the barrel in pursuit of success, we can proceed;
The task is to bore, with a single-point carbide cutter, an extension of the existing chamber along with a leade, such that the 10mm can fit properly. This is way more fiddly and tedious than even careful chamber reaming, with a lot of potential for barrel-ruining screw ups, which is why reamers are preferred in the first place. So builder beware. Since each precision surface is being created individually, there is much more measurement, math, measurement, double checking, and measurement involved. Mostly because you can't see anything that's going on inside the gun barrel while you're doing it (so you have to function as an 'instrument rated' machinist or garage tinkerer)
The good news, is that the cheap carbide boring bar I used (nothing special, basically a round rod with the necessary reliefs & square cutting edge ground, for like 10$ on ebay) bit into the metal like it was nothing at the highest speed my lathe could turn, at cut depths as shallow as .002", and left a very smooth surface. This is the ingredient that's been the bane of so many other conversion attempts, now that aftermarket barrels are gone (lifesizepotato mentions sending his HK barrel to three different smiths for reaming, fruitlessly, before turning to EFK)
The bad news is it is really easy to screw up, unless you've got a very good indicator setup and a thorough process to keep track of everything. In my case, I'm a hackneyed garage tinkerer, so I have to rely on fit-check and simplification of the job more than someone better equipped. Rather than match the very subtle case taper of the 10mm vs 40 S&W chamber, I just cut a straight-walled extension to the original, gradually widening its diameter until an unfired case could drop in/out freely without catching. Rather than cutting the proper SAAMI-profile leade that has a straight then tapered section, I cut a single taper between the start & end points (a tiny bit tighter for certain bullet profiles). My chamber actually ended up a tiny bit deeper than it should be, but it doesn't appear to effect function outside of very unique/odd circumstances (primed, empty cases jump the extractor claw when fired & must be tapped out with a bore rod; live rounds have no such issues or irregularities). In summary, gotta be careful.
The best news is the gun works great so far and is a hoot to shoot. Very much more fun with this slightly more powerful cartridge (which can obviously still get a bit more potent than FMJ practice rounds if needed). The muzzle report, recoil, and authority of 10mm on steel plate seems much more appropriate for the size and weight of the gun. I was single-feeding rounds from my Para P14 mag since I was too impatient to test the barrel to complete the mag mod, but feeding worked just fine. Extraction & ejection were...very aggressive with the standard recoil assembly; MP5 would be comparable. Gonna definitely replace that with a beefier unit so the ejector/extractor aren't working so hard.
TCB
