Curly_

From: Dr. "Curly" Aberdeen, Researcher, Rapid City Ballistics Research Laboratory

To: Caliber Selection Committee, Rapid City Union Ammunition Plant

The .224 ZM cartridge has been revised as per the Committee's suggestions following the first evaluation and the revised technical data package is enclosed with this message.

Projectile construction has been simplified with the deletion of the cup sealing off the lead core, and the rather embarrassing meplat of the original submission has been replaced with a rounded meplat of 0.12 calibers in diameter. The core material has also been changed to a lead alloy with 8% antimony content to increase its hardness and further improve its penetration characteristics. The case rim and extractor groove have been increased in thickness to approximately 0.059" and 0.039" respectively, and the case neck and shoulder have been decreased in thickness to 0.0122". The cartridge overall length has also been increase, allowing for the recommended seating depth tolerance of 0.020".

That is all for now. I will be looking forward to the Committee's evaluation of the cartridge.

Personal Notes:

That goddamn Von Karman ogive. I should never have bothered to make another attempt at it, let alone entertain the idea, even if it would have allowed me to increase the caliber to something a bit more..."palatable" for the Committee. I am not wasting another week on this stupid ogive, unless my colleagues are willing to resuscitate my corpse after I inevitably shoot myself out of sheer frustration.

I present my own cartridge in this competition, the .224 Zoomer Magnum, also known as the 5.7×47mm. In order to increase the number of rounds that each individual Sioux Scout can carry, I felt that a small caliber, high velocity cartridge would be in order, one that would have light recoil while also meeting (if not exceeding) the requirements set forth. I originally started off with a 1.775" case length (said case was designated as "No. 1"), but the case capacity was found to be woefully insufficient for the desired muzzle velocities. A second cartridge case (designated "No. 2"), with the length increased to 1.875", was found to be just the right amount of capacity needed. Two projectiles were also to be designed. The Type A projectile would be lead-cored with a thick, gilding metal clad steel jacket to prevent fragmentation in soft tissue, enhancing its penetration and preventing lead contamination in wild game taken as food. A small cup would seal off the lead core to prevent it from being squeezed out of the base of the projectile. A more ambitious, cost-reduced Type B projectile was also conceptualized, consisting of a gilding metal clad steel jacket, a mild steel core, and an overall reduced weight compared to the Type A. However, the Von Karman ogive required for this projectile proved to be an real pain in the ass to model with what limited tools were available, and for the sake of my own sanity, the Type B was dropped in favor of the tangent-ogived Type A.

Type A projectile, cross-sectioned. Projectile weight was calculated to be 79.5 grains, with a G7 BC of 0.265. Note the metal cup sealing off the lead alloy core.

Complete cartridge, Type A projectile, No. 2 case. The case itself is lacquered steel. Total cartridge weight is approximately 13.66 grams, and the total materials cost is about 5.04680378 cents.

Load data. Recoil is approximately 3.38 ft-lbs from a 10 lb weapon.

External ballistics, with nominal muzzle velocity. Penetration into pine is approximately 16.52 inches at 600 yards, rounded down to the nearest hundredth of an inch.

External ballistics, with muzzle velocity reduced by 200 ft/s. Penetration into pine is approximately 15.40 inches at 600 yards, rounded down to the nearest hundredth of an inch.

5 minutes ago, Sturgeon said:

I'm willing to bet you've read a few of my articles...

I have, actually. Pretty informative stuff.

8 minutes ago, Sturgeon said:

In terms of drop and drift? Yeah, depends on your i7 FF but sure. Energy? Probably not.

The i7 FF was the same as 7N6, which is .929 if Wikipedia is to be trusted. I figured that you'd be able to bump up the weight by making the core a bit like M855A1's (copper slug, steel penetrator) since military 5.45 typically uses a steel penetrator core. Some messing around in Fusion 360 helped confirm that for me. As far as the ballistics go, yeah, the energy is definitely not going to be the same. (Also, I must've misremembered the original velocity I ran it at, it was actually 2,920 ft/s. Still, good stuff.)

4 minutes ago, Sturgeon said:

Bullet doesn't have to be that heavy, but I'm a weirdo .224 cal crackpot. 80 grains would do it.
 

I'm a little lukewarm on 6.5 Creedmoor as a 7.62 replacement specifically because of the limited bullet volume, but it really depends exactly what you planned to use. I personally think US forces are arguably missing out by not having any small (<.50) caliber API, but opinions differ.

I'm pretty big into SCHV rounds myself. Running a 60 grain 5.45×39 at 2,900 ft/s though the JBM ballistics calculator was a real enlightening experience, seeing as it managed to meet, if not outright exceed, M80 ball's ballistics.

Also, what would be a better theoretical replacement for 7.62 NATO, in your opinion?

8 hours ago, Lord_James said:

Conversely, if you’re looking for something more empirical to base your judgments on, you can calculate the energy each bullet has within that range; more energy would imply more penetration for dealing with body armors and punching through make-shift fortifications like stone/concrete, though this is not always the case. 

That's pretty similar to my thoughts on the subject, though I look at things more in terms of supersonic range and sectional density (as well the capacity to use special effects rounds like the Mk 211 Raufoss). Theoretically one could make a .224-caliber cartridge with that matched the external ballistics and sectional density of a 140-grain 6.5 Creedmoor (which according to my math should be about 100 grains), but the projectile would end up being too far long for a standard ball loading, especially if it has to be lead-free.

This came up in a recent discussion I had with a friend over Discord, concerning a hypothetical near-peer conflict (particularly in an urban environment). My friend is of the belief that in such a conflict there isn't no reason to field any small arms (not just service rifles or light machine guns) in anything larger than something like 5.56 NATO, the rationale being that a.) you can carry more of it compared something like 6.5 Creedmoor or 7.62 NATO, b.) you're mostly going to be using it suppressing the enemy so that you can drop high explosives on their heads, and c.) since most combat takes place under 300 meters the extra range would be unnecessary. Is there any merit to this line of thinking? What cases can be made for using more powerful, longer ranged cartridges in SDMRs and (tripod or vehicle mounted) machine guns?