400 Corbon: A cheap man's 10mm

While talking with a friend about the ballistics I was achieving with a super-charged 45ACP load he suggested I take a look at 10mm loads. Searching the literature for 10mm will tell you it's the parent of the 40S&W after being deemed "too powerful".

The 10mm round is interesting in that case capacity is similar to that of the 45ACP but the bullet diameter is smaller (0.400" vs. 0.452"). Consequently, one should be able to build loads with lighter bullets pushed to higher velocities than what one could normally do with 45ACP loads. For example, it would be difficult to build a 45ACP load around a bullet lighter than 185gr.

Now, the availability of new 10mm handguns here in CA is limited, due to the "approved handgun list"; essentially, one is limited to a Glock 20 or 29, and a Kimber Eclipse Custom II or Stainless Target II. But, assuming one already has a handgun chambered in 45ACP and one doesn't wish to invest in another handgun just to experiment with 40 calibre bullets, the 400 Corbon round comes to the rescue.

The 400 Corbon is a now defunct wildcard round. The round uses 0.400" diameter bullets in a necked down 45ACP case. The advantage of a 400 Corbon round over a 10mm round is that with only a barrel change, an existing handgun chambered for 45ACP can be used.

My first experiment with the 400 Corbon was with my existing Glock 21. I ordered a 400 Corbon barrel from EFK Firedragon.

The barrel dropped in with only one minor modification. The Firedragon barrel was a tad too thick in the barrel bushing area and hence the slide would not lock into battery completely.

However, about 10 minutes with 200 grit sandpaper to the first 1" of the barrel from the muzzle, fixed that problem right up.

400 Corbon brass can be made by running 45ACP cases through a 400 Corbon full-length resizing die (and trimming to the correct length, of course). Here, from left to right, is a 45ACP case, a 400 Corbon case and a 10mm case.

Now, for a performance comparison between 400 Corbon and 10mm, I used an additional Kimber Eclipse II chambered in 10mm. I worked up two progressive sequences:

• 10mm -- 0.400" Speer 155gr JHP GoldDot bullets atop Alliant Power Pistol powder; weight varied from 8.0gr to 10.1gr in 0.3gr increments.
• 400 Corbon -- 0.400" Hornady 155gr XTP/HP atop Alliant Power Pistol powder; weight varied from 8.0gr to 10.7gr in 0.3gr increments.

Load data for the 400 Corbon is not readily available, so I was being conservative on the upper powder limit. However, since the 400 Corbon case has a greater capacity than the 10mm case (25.0gr of water weight for the 400 Corbon as compared to 23.7gr of water weight for the 10mm) I felt safe to use a greater upper powder limit for the 400 Corbon than the 10mm. Both sets were built to the same OAL; 1.230".

The above two sequences where then shot through a Chrony and the velocities plotted.

As can the seen, the graph of velocity vs. powder weight of the 10mm is "higher" than that of the 400 Corbon. This implies that the 10mm chamber pressure is higher at any particular powder weight. The obvious reason for the difference in chamber pressures, is that the 400 Corbon case has a larger usable capacity than the 10mm case. The 400 Corbon case is shorter, but both cases have roughly the same capacity. This means that when one builds the two rounds to the same OAL, the bullet is seated further out in the case of the 400 Corbon round. Consequently, the usable capacity is larger.

There are two other potential reasons for the difference in chamber pressures.

1. I used two different bullet types; a Hornady XTP/HP in one case and a Speer GoldDot in the other. The two bullet types, though of the same weight, could result in slightly different chamber pressures.
2. The higher case capacity of the 400 Corbon case may be accounting for the lower chamber pressure. This also implies that the 400 Corbon load has more "head room".

The next experiment to try will be built the two progressive sequences using identical bullets. But the results are promising and point to the 400 Corbon being capable of higher safe velocities with the same bullet.