A: Our M6-series optic mounts are designed to fit GI blueprint carbines, which means they will not fit a Universal (or Plainfield, etc.), carbine unless it was an early production model made with surplus military parts. The commercially made parts (such as the barrels) have different dimensions than the GI ones, and thus would not be able to accommodate the same optic mount.

A potential mount design for non-GI spec commercial carbines runs into several problems: firstly, there was a major lack of consistency in the parts used by the various manufacturers over the decades. Because of this, a functioning mount would need to have a higher rail height to accommodate several possible profiles, and we prefer to design mounts with as low of a rail height as possible. Another issue is that every single stock on these commercial carbines would need to be modified in order for a mount to fit.

Further information about when and why manufacturers changed production: Universal began production in 1961, and kept at it through 1986 (the other commercial brands existed in roughly the same general timeframe). Some of the earliest ones produced were 100% compatible with GI carbines because they were assembled entirely from GI surplus parts, but that soon began to change.

The website M1 Carbines Incorporated explains this in detail:

“More than 426,000 carbines were manufactured by Universal Firearms over a 25 year period from 1961 through 1986. This was 10 years longer and more than three times the quantity manufactured by any other commercial carbine manufacturer. In the beginning the first Universal Firearms carbines were 100% compatible with surplus GI carbine parts, their carbines included many of these parts. As with every other commercial carbine manufacturer, as surplus GI carbine parts became scarce, Universal Firearms began using commercially manufactured substitutes. Unlike other commercial carbine manufacturers, some of the commercially manufactured parts used by Universal Firearms were eventually compatible with the Universal Firearms carbines only and not interchangeable with their surplus GI counterparts.

In 1967 beginning with serial number 100,000 Universal Firearms implemented a major design change to the carbines they produced. The design had been patented by Universal and the majority of parts were no longer compatible with their GI counterparts. The design change was significant enough that the carbine it produced was no longer an “M1 Carbine”. What makes a carbine an M1 Carbine is the use of the design and parts of the original U.S. M1 Carbine, as set forth by U.S. Army Ordnance in the 1940’s and early 1950’s. The Universal Carbine retained the overall outward appearance and ammunition of the U.S. M1 Carbine, but the internal design and parts were a hybrid replica of the M1 Carbine.”

Thus, if you happen to own a Universal carbine, you could take it to a local expert and have him determine whether your carbine has a GI barrel or one of the commercially produced barrels. Remember, none of the commercially produced barrels are compatible with our optic mounts, and most Universals have commercial barrels.