I believe that the plural of FTE is FTEs. It is being used as an acronym and the plural of that acronym is FTEs.
Expert linguists make a distinction between words like "FBI" and "DARE." The type you pronounce as a word, like scuba or laser, is an acronym. The type you read out as a string of letters, like "FTE," is an initialism.
Now, this comment here is rather important:
When those gun savvy folk answer, I could care less about their grammar and spelling. If the info is correct and I comprehend what they are trying to convey, I'm happy.
Massive disagreement here. When somebody says something about firearms, offers some kind of information about them, you have to ascertain how reliable and trustworthy that datum is. There are a few ways to do that.
You might take into account how long the person has posted, how reliable they have been in the past, etc. Failing that, the person's literacy is a gauge of believability. Say you ask a question like, "I'm thinking of getting a Ruger MKII or MKIII. Is one better than the other?" And you get this in response:
i thk u shud get a Mkii cuz its ez to dissasembul.
Well, I don't know about you, but I'd take that remark as coming from a complete idiot and would ignore it. Even if it were true, I'd have to assume that the writer was a mouth-breathing dingbat and would need confirmation from someone who can write in English.
This doesn't mean that every comment here has to be made in impeccable English. Internet communications are often made at speed, and there are lots of jokes that revolve around common errors like "teh" and "pwned," etc. Still, writers ought to make some effort at spelling and punctuation, some minimal attempt, however meager.