I'm aware of ammunition used for training troops to keep their heads down, called Overhead Fire. Fixed MGs were fired while soldiers crawled along under the fire. My question is why was a different round required, and what was different about it? Conceptually, it seems that standard ball would be OK.

U.S. manufactured OHF ammunition had tighter accuracy requirements and OHF tracers specificly used a different moisture seal between the tracer pellet and the powder in the case, plastic instead of copper, I think. Apparently the copper discs could detach and hit the soldiers when the rounds were fired. I'm not sure if the OHF ball was specificly made as such or was drawn from lots that were more accurate than usual. There is also a specification for higher accuracy requirements between regualr box-packed ball ammunition compared to linked-pack 7.62x51mm ammunition. Again, produced that way or selected after manufacture? I'm sure someone else had a better answer...

AKMS

Good answer. Common "internet" wisdom says that MG ammo is intentionally less accurate so that you get a better spread. I usually get scoffing comments when I give a variation of your reply.

There was an experimental 7.62x51mm NATO overhead fire ball round, XM172. It had a solid turned GM projectile. As far as I know they must have found that standard ball rounds were sufficient, since this was only an experimental cartridge.

Here's a photo of one along with a 7.62x51mm NATO overhead fire tracer:


7.62x51mm NATO XM172 Overhead Fire Ball


7.62x51mm NATO Overhead Fire Tracer

Conversely, some lots of German ammunition were labeled "not for overhead fire or firing through breaks in the line<" probably a result of some lots with somewhat erratic accuracy.

U.S. .30-06 ammunition of the WW2 era was apparently not specifically made for overhead fire training, but rather all was considered acceptable, unless "downgraded" for some defects (mainly due to age) and suchlots were supposed to be marked "Not for overhead fire" for the protection of troops.

I am not sure if this applied only to training exercises involving overhead fire while crawling under barbed wire, etc, or also applied to firing over the heads of troops in positions ahead of the machine gun(s) in combat situations, or training exercises.

IN British Service, it was not the ammo, but the Gunbarrels of Vickers Guns which were segregated for Overhead Fire, due to the different Barrel Erosion Patterns between Mark VII Cordite, and Mark VIIZ and Mark VIIIZ; Cordite ( double base) and "Z" Nitrocellulose (Single base) gave different "flame erosion" characteristics in the gun barrels; so much so that barrels used initially with Cordite, were not suitable for Overhead fire with 7z or 8z; and Barrels were so marked.

The Only other marking was "Not suitable for Overhead Fire" on specific lots of Mark 7 and Mark 8, due to age, climatic conditions etc.

I remember at JTC Canungra, well into the 1980s, the OHF Vickers guns were always fed Cordite Mark VII, and when the Aussie 1959 ammo ran out, they imported Mark 7 (Cordite) from India, K^F 1971 etc.

regards,
Doc AV
AV Ballistics.

I've recently seen US ammo data sheets on the 7.62mm M62 tracer which describe an Overhead Fire Version of the round (same designation) distinguished by an orange rather than red bullet tip (IIRC).

In much of the M-62 OHF, the foil moisture seal in the base of the bullet has been eliminated entirely. I am quite fond of these projectiles as the can be bumped to .323" (at about 100,000 psi) in a 10s PF die. They then make great tracer bullets for 7,92 MG's. JH

Oops, my memory playing tricks again, got them the wrong way round: standard M62 tracer is orange-tipped, Overhead Fire Mission M62 is red-tipped.

Tony,
Actually, that is the only way it makes sense: The normal 7.62X51 tracers (orange color code on bullet tip) are designed to start trace at some distance from the gun, and sometimes a metal cup in the rear of the case provides this delay, as it must be "burned through" to ignite the trace mixture. In the described training "overhead fire" the guns are mounted fairly close to the "crawl space" the trainees are traversing, and a "delay tracer" would not have the time to ignite, and would provide the troops no input.

By using an "immediate ignition" tracer such as the predominant tracer types from WWII, the trace would burn as it passed over the trainees, providing some stimulus. Immediate ignition tracers have tradionally been identified by the red bullet tip color.

The worst option would be to back up the firing point from close proximity to the training or "crawl" lanes: This would allow a chance for greater bullet, dispersion, greater bullet drop, less accurate fire, and less intense input to the trainees from the noise, flash, etc. resulting from the firing. So, we would have less training gain with greater risk to the troops.