There were many difficulties during the initial phase of production. The first production models had stamped sheet metal receivers with a milled trunnion and butt stock insert, and a stamped body. Difficulties were encountered in welding the guide and ejector rails, causing high rejection rates. Instead of halting production, a heavy machined receiver was substituted for the sheet metal receiver. Even though production of these milled rifles started in 1951, they were officially referred to as AK-49, based on the date their development started, but they are much widely known in the collectors' and current commercial market as "Type 2 AK-47". This was a more costly process, but the use of machined receivers accelerated production as tooling and labor for the earlier Mosin–Nagant rifle's machined receiver were easily adapted. Partly because of these problems, the Soviets were not able to distribute large numbers of the new rifle to soldiers until 1956. During this time, production of the interim SKS rifle continued.