Pattern Web Equipment led the magazine to be shortened to its final capacity of thirty-four rounds; when introducing the Mk 7 Pistol variant in 1983, Sterling produced magazines with capacities of ten and fifteen rounds for that weapon, as well as introducing "twin-stacked" versions of these and the original thirty-four round magazine. The Sterling magazine is said to be one of the best ever designed. While Sterling provided magazines for British Armed Forces weapons, with these being designated as "Magazine, 9mm, L1A1", cost issues led the Ministry of Defence to pursue the design and production of a non-Sterling L1A2 version which used a stamped and electrically seam-welded two-piece construction with a singular positioning lug and was 9.8" long down the rear spine compared to the 9.6" of Sterling-produced magazines. Production Sterlings retain the earlier Patchett gun's compatibility with Sten and Lanchester magazines; while the Lanchester gun is theoretically capable of using Sterling magazines, the same cannot be said for the Sten gun since a Sterling magazine intrudes " further into the bolt way than a Sten magazine and so attempting to fire a Sten gun with a Sterling magazine fitted would cause the breech block to foul the rear of the magazine, while attempting to counteract this by withdrawing the magazine by " would merely result in the magazine's top round being out of alignment with the breech block by ". This issue is not present on Sterling guns fitted with Sten magazines; while the magazine will be " further away from the breech block's centre line, the top round remains in line with the breech and so can be reached by the Sterling's more robust feed horns. The Sterling gun and its magazine were purportedly designed this way in order to prevent a situation where, instead of buying the actual Sterling gun, prospective users might only buy its magazines and then use these with their pre-existing Sten guns. The Sterling employs a degree of what is known as Advanced Primer Ignition, in that the cartridge is fired while the bolt is still moving forward, a fraction of a second before the round is fully chambered. The firing of the round thus not only sends the bullet flying down the barrel but simultaneously resists the forwards movement of the bolt. By this means it is possible to employ a lighter bolt than if the cartridge was fired after the bolt