Extended Magazines for Bolt Action Rifles
The bolt-action rifle was the premier weapon of the early 20th century. With virtually every country in the world using an Arisaka, Lebel, Mauser, Mannlincher, Enfield or Mosin designed box fed weapon, the foot soldiers of every continent stood neck and neck in the small arms race. Once the balloon went up in August 1914 and World War one erupted, these bolt guns suddenly could not be upgraded and modified fast enough to compete with a modern battlefield controlled by belt-fed machine-guns.
The British, with a professional pre-war army built on long serving regulars, had already adopted the Short Magazine Lee Enfield (SMLE) whose 10-round detachable magazine doubled their rate of fire. Germany responded by retrofitting large "Trench" magazines to their Gewehr 98 Mauser rifles. Holding 25-rounds in a steel box, it replaced the standard factory installed five round internal magazine and hung from the bottom of the rifle prominently. The downside was that the rifle could no longer be fired in a true prone position, but this was not a problem on the trenches of the Western Front.
German WWI Trench mag. The key affixed to the magazine is inserted when the mag is loaded and detached from the rifle to keep the rounds secure.
The United States Army came to the trenches after 1917 with their own modified bolt-action rifle, the Pedersen device-equipped Enfield 17's and Springfield 03's. Some Springfields were also issued 20-round 30.06 magazines. If you can find either the 20-round extension mags or the 40-shot Pedersen devices today they are worth four-figures easy.
The WWI US Pedersen device could spew 40-rounds at the pull of a trigger.
Trench Mags Today
Today original Trench Magazines for Mauser rifles are rare and expensive. However, there are a few work-arounds for this. One is to take vintage 25-round WWII era MG13 magazines and modify them slightly for use with any number of 8mm-sized Mauser rifles including VZ-24s, Yugo's, Persians, FNs, and Turks. This involves some fitting and feed lip work. These MG13 magazines are cheap and well made. Companies such as IMA offer them for as low as $30.On the upside, they work, are reliable, and will turn your bolt Mauser into an ersatz assault rifle. On the downside, sometimes the springs are shot and if you are going for a reenactor's piece, you are incorrect. These can also be used on Hakim rifles if you dremel a magazine catch into the magazine.
K98 with MG13 magazine attached.
For those reenactors and purists, there are new construction authentic reproduction magazines distributed by Sarco http://e-sarcoinc.com/mauser20roundtrenchmagazine.aspx and Keep Shooting for about $70. They are blued steel with the feed lip-retaining pin and are even marked authentically. These are made for the old WWI era G98 and will fit that model without too much of an issue. Simply remove the floor plate and original follower and spring, and then click the new magazine into place. Then remove the pin and position the new follower and spring. If you try to use these on any other 8mm Mauser, good luck. They can be modified for K98s and others but many who do report feed issues unless the magazine is almost completely rebuilt.
At least one manufacturer is working on a 10-round extended magazine for the Mosin Nagant M91-series rifle. The magazine seems to work well on the video of the prototype and if it can be made for less than the price of the firearm's current value, could prove to have lots of sales, as the Mosin is one of the most popular surplus rifles available in the country.
Other exotic builds include using surplus USGI BAR magazines for Egyptian FN-49 in 7.92x57mm rifles; CETME mags for 7.62x51mm NATO chambered Israeli K98s and others. Odds are, if you have a former military bolt gun, you can fit an extended magazine to it if you are crafty enough.
Some standalone custom builders like 7.62 Custom, produce ground up models of legacy bolt guns supped up with flash enhancers and trench magazines. Their Zombie Slayer model, while pricey, is certainly an example of this and an inspiration for weekend builders everywhere.