On Reliability of Drum Magazines

By olegv published on in Ammunition, General, News

Drum magazines have long had a bad reputation. “We found many Angolan and Cuban soldiers dead, with jammed RPK drums in their rifles” said a South African veteran. “PPSh drums had to be down-loaded by a few rounds, usually had to be fitted to individual submachine guns, and jammed more often than box magazines” wrote Soviet veterans of WW2. And yet drums persist in weapons large and small — have you ever wondered why?

Puma PPS50 drum byBlack Dog Machine keeps each cartirdge separate and not under pressure.

Puma PPS50 drum by Black Dog Machine keeps each cartridge separate and not under pressure.

MD Arms 20rd drum for 12ga Saiga has same featureL it may be left loaded long-term without defornation of the ammunition from spring pressure.

MD Arms 20rd drum for 12ga Saiga has same feature. It may be left loaded long-term without deformation of the ammunition from spring pressure.

Drums actually predate semi and full auto firearms. Several designs, gravity fed or mechanical, were in use with the Gatling guns. The early pan designed for the .303 Lewis light machine gun copied the mechanical arrangements of the Gatling. A ratchet in the receiver activated the magazine rotation with each stroke of the piston. That avoided dependance on the rather unreliable clockwork springs which were the other typical motive power for drums. Clockwork rotary magazines are almost as old: the Krag-Jorgensen rifle used one, and so had Mannlicher-Schönauer. Ruger 10-22 and Steyr SSG69 are direct descendants of the latter design. Its major advantage is individual handling of cartridges, which protects them from deformation. With shotgun magazines, such as the MD Arms drum, the freedom from deformation is an important feature, as plastic hulls can go out of round in longer box magazines. It’s equally important for the 22LR cartridges which are relatively delicate: only the few rounds inside the feed tower are under spring pressure, the rest are safe between the pawls of the drum ratchet.

The “snail drum”was the first widely used high-capacity clockwork-driven magazine, first used with Luger pistol and later with MP18 submachine gun. A cumbersome device of imperfect reliability, it was replaced with stick magazines in the successor MP28. The recent MGW 90-round drum follows that concept. The German machine guns of the 1930s improved reliability and feed rate by using two springs, giving the magazines a distinctive double-drum (doppeltrommel) look. MG15 aircraft gun ran at over 1000rpm and the drum kept up. That dual-spring approach is still used today with Beta and Armatac dual drums.

PPSh drum nominally held 71 rounds but loading 65 helped to avoid jams.

PPSh drum nominally held 71 rounds but loading 65 helped to avoid jams.

Chinese 75rd RPD drum improved on the Russian version by using multiple pawls to move the ammunition.

Chinese 75rd RPD drum improved on the Russian version by using multiple pawls to move the ammunition.

Thompson, Suomi K31, PPD and PPSh drums all used a floating follower set inside a spiral track. Thompson magazines (and later Chinese RPK drums) separated cartridges into clusters to reduce friction. The others, along with the derivative Russian RPK drums, did not. With the limited power spring, friction became an issue. With the Thompson drum, better reliability was bought at the cost of not having all cartridges under pressure and rattling against each other.

Better spring metallurgy and longer feed towers (to make feed lip position repeatable) make modern drums more reliable. Better casing materials and designs also make them very durable: Armatac’s designer Mike Snow demonstrates how tough his creations are by forcefully bouncing the loaded drum several time off a concrete pad, then firing off the entire 150-round load with no stoppages. While bulkier than individual 30-round box magazines, drums are more space-efficient when heavy volume of fire is needed. Compared to belts, they allow for lighter, simpler feed mechanisms on the guns. Drums also keep out dirt better than belts do. This is why Ultimax100 and MG36 light machine guns both use drums instead of belts.

Heavy-barrel FAL by DSA with 100rd Beta drum

Heavy-barrel FAL by DSA with 100rd Beta drum

Anderson AM15 carbine with 150rd Armatac drum

Anderson AM15 carbine with 150rd Armatac drum

In civilian use with semi-automatic rifles and shotguns, drums are best suited for defending fixed positions, such as homes. Where light weight and mobility are less important than being able to turn back a determined assault, drums have a place in our ready kits. In such cases, heavy barrels are advisable as well. Coming back to the bad reputation of drums — the two examples mentioned were both mediocre designs produced to indifferent quality and usually badly maintained. Suomi drums, while internally similar to the PPSh design, were made to much higher tolerances and were more reliable than the quad stack box magazines also fielded by Finns. In case of the Cuban and Angolan troops, even such reliable stalwarts as the AK47 feel to poor maintenance and lousy training. Judging modern drum designs by those examples is not very useful.

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