Depends on the design of the weapon, but yes it is possible. Recommended is a completely different story. You may find the slide of the weapon flying into your face when it fails from the rapid fire hammering it wasn't designed to take or some other kind of failure.

Sometimes ammo capacity, and usually recoil become issues. To compensate for that you use a larger mag, put a foregrip or a collapsing stock on, and maybe a sling.

Then you call it an SMG, or perhaps a Machine Pistol.

Or someone with a Glock living in a jurisdiction where it is legal for them to own a FA and they have enough brain power and dexterity to mailorder the BF/FA replacement backplate for the slide. If you are capable of ordering it you are capable of installing it.

Or you just buy the Glock 18 itself, or the 18c which has barrel mounted recoil compensation, which is a full auto version of the Glock 17.

mfb and I are talking about Real weapons, which is what I think Moon-Hawk was refering to being able to change. I think Moon-Hawk wanted to know why you couldn't just do this in SR.

The SR3 book Canon Companion had a section for modifying an existing weapon. But it only allowed SA/BF SMGs and machine pistols to be converted to SA/FA weapons. It didn't allow SA-only to be converted into BF or FA. Balance is likely the reasoning, but IMO I think all they needed was to give an extra recoil penalty just like they did for the option of converting from BF to FA.

EDIT

If your GM is ok with the concept there is no real need to wait for Arsenal. Just have an Armorer Extended Test of 10/1 hour to convert most SA weapons to BF or FA. Only Armorer Extended Test 5/10 minutes if you instead manage to find a blackmarket kit. Then give the shooter an extra -1 or -2 die recoil penalty on top of whatever recoil they normally would have for BF/FA.

It depends very heavily upon the weapon, even for semi-autos.

Glocks and Berretta 92s are apparently relatively easy to modify for a selectable FA mode. There are parts available that are designed to do this. Fabricating them from scratch would obviously take more knowledge and time. 1911s can be made FA only very easily. But not just any gun can be converted as easily as these. I do have rules on my site especially for the FSS-G modification.

Most AR-15s (like those that Bushmaster makes) would require some tooling (at least a milling machine and a drill press and correct size bits for each) and some knowledge of parts fabrication and assembly to convert from SA only to SA/FA. The lower receiver, bolt carrier and selector have to be modified and a few additional parts (auto sear, auto hammer) have to be fabricated. Adding burst fire would certainly increase the difficulty by a substantial margin, requiring more knowledge of engineering and the timing involved (if you were to fabricate the parts yourself, that is).

As a general rule (SR3), I would say that for modifying an SA weapon to SA/FA should require a Firearms B/R test with a TN of 5 with a base time of two hours, the use of a machine shop, and 100 for stock.

no kidding? i thought it was just a matter of changing out some parts.

Well, kind of. There are two ways of going about it, really.

In semi-auto AR-15s, some of the area within the lower receiver that would normally be occupied by the auto sear and auto sear spring (and by that I mean the actual Armalite-engineered parts designed to work properly in an M16/M4) is filled by part of the lower receiver itself, which is a chunk of forged aluminum. When a company makes AR-15 lower receivers (as opposed to M16/M4 lower receivers), they just leave a couple of milling and drilling operations out, really. In the US, that's mandated by the BATFE specifically so that AR-15s can't be easily modified for full auto. So you'd have to mill that extra material out and drill a hole through the receiver for the auto sear pivot pin. But other than that, it is just a matter of changing out parts (the trigger, disconnector, hammer, selector switch and bolt carrier). Here's an article that will give you some visuals and a better idea of what I'm talking about.

That said, there is a device called a drop-in auto sear that can be installed in some semi-auto AR-15s (along with the auto trigger group, selector and bolt carrier) to make them SA/FA capable. However, from what I've read, this is a fairly less-than-reliable, "desperate man's" way to get that kind of functionality. Also, these days lower receivers are usually manufactured to prevent even a DIAS from being installed. So if you must have full auto, you're more than likely going to have to spend time machining. You might as well do it the right way (or maybe that's the really wrong way you bad person you way). The penalty is going to be the same.