40 S&W chamber with 10mm length magazine?


February 13, 2013, 06:35 AM
This photo is of a 200 grain 40 caliber bullet from a mold I designed on the Mountain Molds website. My design goal was to create a heavy 10mm bullet that took up the least amount of room inside the case as possible. In other words, I tried to design it to be as short for the weight as possible. It's a 75% (.300) meplat and is significantly shorter than the RCBS 200 grain design I was using previously.

Due to the relative cost and scarcity of 10mm cases I bought a Lone Wolf 40 S&W barrel for my Glock 20. That opens up some interesting possibilities in that the barrel will chamber a round with a COAL that would never fit a standard 40 S&W magazine but will easily fit inside the longer 10mm magazine of the G20. The photo shows a dummy round with a boolet loaded out much farther than I could get away with seating it in a 10mm case. I arrived at this length by gradually seating it deeper and deeper until the head was flush with the barrel hood when dropped into the chamber of the 40 S&W barrel. The dummy round fits into the 10mm magazine at about the same length as a regular 10mm round does.

This raises the possibility of maybe getting heavy bullet performance from the 40 barrel that more closely approximates 10mm performance since there's now more powder space available due to the long seating length. I'm going to experiment (very cautiously, I assure you) with seeing if I can exploit the extra COAL. Bullet setback is a fear, of course, but by concentrating on slower powders at first I might be able to prevent that possibility by loading to near 100% density.

Surely I'm not the first person to contemplate going down this road? Anybody have any results they would like to share about shooting "40 longs"? I should add that the chances of me shooting these 40 longs in a regular 40 are nil, since they won't fit in the magazine and also because I don't own a 40 other than the Glock barrel.


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February 13, 2013, 08:26 AM
Interesting concept. Have you loaded any of these bullets to SAAMI spec and tried them in a factory .40cal pistol? I know you don't own one, but lots of your buddies probably do. There is a lot of 200-gr jacketed data out there, but far less 200-gr lead data. Do you have specific powders in mind?

Be aware that using that extra powder space means putting more powder, and potentially a lot more pressure, into a .40 case and barrel than they were meant to manage.

Please do let us know how the trials go, and be cautious!

Jim Watson
February 13, 2013, 09:40 AM
As an economy measure, fine.

I would not push it to where it "approximates 10mm performance."
That would be in unknown territory for pressure in a weaker case.
I'd settle for reliable function.

February 13, 2013, 10:07 AM
I would briefly point out that .40 S&W brass is no weaker than 10mm brass, not at all. If anything it's stronger.

What is the OAL of that load? Likely the chamber of the barrel is cut to accept "spec" .40 S&W ammo, but if you can load it longer that's great.

In my Glock .40 I use a KKM 6" barrel and it will allow me to seat .40's out to around 1.160", the problem is with the magazines because while some 1.160" will fit, usually it has to be around 1.150-1.155" to fit in the magazine, depending on bullet profile. They are longer yes, but not long like some of the 1911 guys who can get away with loading them to 10mm OALs. While I don't know what your OAL is, if the OAL is the same as the 10mm then you can use 10mm data.

With the right powder in the 6" KKM I've got both Beartooth 200gr WFNGC (shortest 200gr hardcast I've found) and 200gr Hornady XTPs over 1200 fps average. However those aren't loaded long, but rather to around 1.125-1.130". I have played around with some 220gr hardcast seated to around 1.150" and can get about 1100 fps out of those. My longer loaded rounds are limited to about 1.160" though.

February 13, 2013, 10:11 AM
Check out the Brian Enos forums for Tanfoglio/Witness guns. There was a time (before redesigned mags) when guys had to load long to get their Witness .40's to run in competition.

And Clark, the mad scientist* who posts here, tells of testing 10mm and .40 to failure. He claims that .40 can actually be loaded hotter, in part by loading long, by virtue of the smaller primer pocket making .40 brass inherently stronger. Going down that road is definitely heading into the wilderness... But if you're interested, check out some of his posts here.

* Said with great respect

February 13, 2013, 11:16 AM
I'm not interested in getting up anywhere near the failure point. I've already had to replace the frame on my G20 because of a case head failure from a bad batch of brass.

I wish I could do this experimentation in my 1006 with a 40 barrel but nobody makes them. A case head failure is much less expensive with a steel framed gun...but I'm not interestd in getting anywhere near that point. Reasonable velocities and reliable function are mostly what I'm looking for, with reasonable being defined in terms of not exceeding established pressure limits for the 40 S&W case. I'm hoping to exploit the newly available case capacity by being able to use slower, bulkier charges to keep pressures within the established limits of the case but maybe push a heavier bullet a little faster than I could if it had to be seated to 'normal' depths.

February 13, 2013, 12:28 PM
CAUTION: The following post includes loading data beyond currently published maximums for this cartridge. USE AT YOUR OWN RISK. Neither the writer, The High Road, nor the staff of THR assume any liability for any damage or injury resulting from use of this information.

If you load to 10mm COAL your pressure will be no different between 40 brass and 10mm brass as case volume will be equal. At least as equal as the variations between different manufactures. You should have no problems using 10mm starting loads and working up from there.

February 13, 2013, 03:15 PM
I've seen where people have done this before. The .40 cal brass has a stronger case head because of the small primer pocket. Plus it's WAY cheaper than 10mm.

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