In another forum a discussion about the Coonan 357 Mag 1911 eventually came around to 10mm as an alternative. I did some comparative analysis that I thought this group would appreciate:
A 5" or less breech face to muzzle comparison gives the 10mm auto a performance advantage over a 357 magnum. Physics is a relentless girlfriend.
F = P * A
For 357 magnum that means F = 35000 PSI * ((.357/2)^2)*3.1415 = 3,503.334 lbs-force
For 10mm Auto we get F = 37500 * ((.400/2)^2)*3.1415 = 4,712.250 lbs-force
It isn't even a close race in "short" (5" or less) barrels.
Some background as this was a bit of a slog to extract. First I determined comparable bullets by using sectional density to normalize for the difference in diameter: 125 grain 357 (.140) vs 155 grain 10mm (.138). I then went looking on the Ballistics by the Inch website for data. They had solid data for Federal 125 JHP and Corbon 125 JHP that aligned with the best velocities that Underwood ammo advertises. But Corbon loads for 150 grain and 165 grain 10mm were basically baked over 40 S&W data, acheiving velocities as 5" that are closer to what a 200-220 grain 10mm full power load can achieve.
So, I took an average increase in velocity from the BBTI 165 grain data and applied it to the Underwood muzzle velocity for a 5" barrel and extrapolated the velocity for the other barrel lengths. I cross checked my process against other BBTI bullet weights in 10mm and had an accuracy of a couple percent, so I used this derived data for the comparison.
To be sure I had not introduced an unfair advantage to the 10mm I applied the same extrapolation method for the Corbon and the Federal 125 grain velocity data, and then for each barrel length I used the fastest of the four velocities for the 125 grain 357 mag from the actual corbon, actual federal, derived corbon or derived federal. If there is a finger on the scale here it is for 357, not 10mm.
The results:
| Barrel Length | 357 Mag Velocity | 10mm Velocity | 357 Mag Energy | 10mm Energy | Delta |
| 18 | 2113 | 1731 | 1240 | 1032 | -17% |
| 17 | 2067 | 1729 | 1186 | 1029 | -13% |
| 16 | 2119 | 1727 | 1247 | 1027 | -18% |
| 15 | 2069 | 1719 | 1188 | 1017 | -14% |
| 14 | 2054 | 1717 | 1171 | 1015 | -13% |
| 13 | 2024 | 1705 | 1137 | 1001 | -12% |
| 12 | 1994 | 1690 | 1104 | 983 | -11% |
| 11 | 1978 | 1675 | 1086 | 966 | -11% |
| 10 | 1943 | 1662 | 1048 | 951 | -9% |
| 9 | 1901 | 1641 | 1003 | 927 | -8% |
| 8 | 1851 | 1614 | 951 | 897 | -6% |
| 7 | 1784 | 1586 | 884 | 866 | -2% |
| 6 | 1715 | 1550 | 817 | 827 | 1% |
| 5 | 1614 | 1501 | 723 | 776 | 7% |
| 4 | 1511 | 1441 | 634 | 715 | 13% |
| 3 | 1257 | 1321 | 439 | 601 | 37% |
| 2 | 1003 | 1211 | 279 | 505 | 81% |
So, if 10mm has such a huge extra force budget why does it loose the long race? Case volume. The 357 Magnum has 8.5% more of it. This comes back on the 10mm two ways.
First, a bigger energy budget means the 357 can apply the force longer than the 10mm can. And work is force * distance. Given a longer distance the 357 pulls ahead.
Second, P1V1 = P2V2. Since the 357 Magnum has a higher initial volume, it has a flatter pressure curve over distance. For longer barrels this again contributes to more work done.
So, for a breech to muzzle of 5" or less, pick 10mm for a BIG performance improvement. Between 5" and 7" it is a tie, and after 7" the 357 Mag runs away with the race.
Interesting data... Most loading data for the 357 Magnum is from a 10" barrel vs the 5" for 10mm.
I have always taken that to heart knowing that my 6" 357 Magnum was way shorter than that of the 10' test barrels or firearms used.
Taking that into account back in 1979 / 1980 is why I started using Blue Dot instead of the H110/W296 & 2400 for the loads. To me that yielded better performance for my pistol and used less powder by weight.
One load I really liked was the RCBS 180 Silhouette Jacketed and cast with gas check. (this bullet is loaded & crimped in the cannelure. It is too long for some revolver cylinders and too long to feed from the tube magazine of my Marlin Lever, so they get loaded as drop in.)
The amount of bullet inside the case was the same as my 166 grain SWC and the slight weight difference wasn't a factor.
Caliber – 357 Magnum
Powder-Blue Dot 11.0grs
Case – Mixed
Primer – CCI 500
RCBS 180gr Gas Check Silhouette
COL – 1.660" Sized .358"
Velocity 1320 feet/per sec
(https://i.postimg.cc/GmtWFKvV/Speer357-180-Sil.jpg) (https://postimages.org/)
Because I had a good concept of Blue Dot and used a lot of it for 357mag/38spl, 44mag/spl as well as 12 ga, it was only natural for me to develop loads for my 10mm at the very start of 1990.
Back then the 175 grain JHP and 200 grain FMJ's were the common bullets, and I was loading those with 10.4 grains for very accurate ammo. I also put the 357 mag away and carried the S&W 1006 which held 10 rounds.
That also translated into using Blue Dot for my cast bullets as well...
(https://i.postimg.cc/pTq8Gpdp/10mm-Cast-Bullets-zps0b199796.jpg) (https://postimages.org/)
Later I bumped up my load for the 10mm using the Hornady XTP's with 10.5 grains.
As time went on, I used a whole host of other powders for the 10mm.
There were some good velocities to be had with some of those powders...
All in all, 10mm AUTO proved to be a great choice and still has my favor for what it brings performance wise.