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Converting a TM 552 to use M4 mags


j33v3s

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As in the title really.

 

Before anyone says "King Arms Sig 556", I don't want to buy a whole new rifle, and then have to tune it to the point my TM 552 is at. I also dislike the way the M4 stock mounts onto the gun, partly because I don't like the "converter block" as I call it (it puts the stock in the middle of the rifle, I'd much prefer it being a bit higher up), also because I feel it makes the sig look far too M4 like. I also like my current stock.

So essentially I just want to be able to use M4 mags in my 552.

 

I know there was a GWS conversion kit a long time ago, but being limited edition I can't find it in stock anywhere, and it's fairly pricey anyway. Plus I don't think it was ever produced in 552 length, and it comes with all the extra bits I don't want.

I've emailed the guys who made it anyway, as the website is still up, so we'll see if I get a reply from them or not.

 

 

Onto the conversion:

 

My thoughts on this so far is to see if I can get hold of the lower receiver and the feed tube part from either a GWS kit or the King Arms 556. Before I do this, I want to know if anyone has had a look and can tell me how easy it would be to put the lower receiver onto the TM 552 - am I right thinking it would just be totally drop in?

 

That's it for now really, if I manage to get hold of a lower receiver really cheaply, I'll give it a go myself and update here. But if anyone else has ever done this, or even just compared the two, I'd appreciate the input!

 

Thanks

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You should post pics of the various parts.

 

Measure the feed port on the m4 mag vs the sig 552 mag in relation to distance from front and back, and the overall measurements of the magazines exteriors. This would help us understand if its easy or hard to make an adapter or attach a mag well to the lower.

 

You want this to be permanent or temporary and removable?

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Not fussed, if it's easy enough to make reversible then all good, but TBH I'd be getting a whole load of M4 mags, and assuming it worked fine I'd get rid of the SIG mags.

 

I don't have any M4 mags at the moment as I have no M4 compatible rifles so unable to compare. I do know from looking at take-downs guides that the King Arms 556 uses an M4 hop unit which is shimmed from the gearbox, whereas the GWS conversion kit did have a feed tube that allows the M4 mags to feed into the default 552 hop unit.

From memory, the M4 mags feed lips are at the front of the mag, while the 552 mags are in the middle.

 

My main motivation is easier to find pouches, higher capacity mid caps without looking stupid (the mids for the SIG are far too long) and lower profile - as a result of the thinner mags, and needing less of them.

 

Been doing a bit of research - the 552 hop unit is held in place primarily by a screw in a plate at the top of the mag well. This is what keeps the "tension" on the gearbox, it also serves to hold the hop unit straight. The 556 doesn't look like it has this plate at all, however, a thin one could potentially be mounted, although there wouldn't be any room for a nut and bolt or a screw (if the space internally is the same as in the other 55x models), it'd have to be glued in place somehow which I don't think would hold up too well to the force of repeatedly inserting mags - or the fact it'd have to hold the tension of the hop unit.

In terms of tension against the gearbox, I could use a spring for that job (similar to the g36 hop units - as they're pretty much identical in design).

 

The main difficulty here would be manufacturing the feed tube itself. After that I think I'd need a 556 lower receiver and have a play with it to get everything securely mounted - which would take me back to the original issue - will it drop straight onto the existing upper receiver/stock/fore grip mounting points. Unless anyone's done it before, probably the only thing to do is get one and see!

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Mod some 552 mags:

 

Easy:

make them shorter, mod exterior thinner. Might provide the results your looking for and is a much easier and CHEAPER intro to modding. get some very cheap 552 mags and mod them, try to find some broken ones and fix them to understand them, which makes modding them much easier.

 

Moderate:

mod m4 mag to take 552 mag internals, mod 552 mag catch onto m4 mag. give mag a wider lip or make a simple insert for the magwell to fit m4. Hardest part making 552 mag catch on m4 mag. Dubious results even if done well, may not slide out of pouches well with 552 mag catch.

 

Hard:

Mod mag well and hop unit to take m4 mags, seriously, 556 or just get an m4. This will take more time money and effort then you are aware of. It is a serious mod, and for dubious reasons. Even done extremely well it wont be an AEG that resembles anything real and will probably not be what you wanted. Much easier would be to get a 556 and see if it will take the normal 552 stock, that would likely be an easy mod if it isn't drop in functional.

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Yeah, I'm a week late to the party...

 

Short answer: It wouldn't be easy, if it's even possible.

 

Longer answer: A KA SIG 556 lower would not be a drop in fit with the TM. A Cybergun one might work (since I think it's based on the design of the GWS kit, since that version actually uses a normal SIG hop up chamber), but the KA won't. The length of the receiver is different. From the pistol grip (or trigger, which basically relates to the positioning of the gearbox) to the front body pin is a different length, likely due to the need to have the M4 magazine positioned farther forward because the back of it would normally interfere with the V3 gearbox. Whether or not that's to spec with the real rifle is a different matter, but for the KA, that spacing is different. As such, the upper receiver is also a different length, so either you're going to have a gap of space, or you're going to have to cut off part of the receiver to allow it to fit. Then, you'd need to deal with making everything mount properly (because the way the stock mounts on the 552 is different from the 556, and the same goes for how the upper and lower receiver are fixed to each other (in the TM and JG). I was checking dimensioning relative to the JG, but given that it's a direct clone of the TM (to my knowledge), it should be analogous. I also checked compared to G&G SIG receivers, and it has the same spacing. Though it has differences inside the receiver, the overall dimensions are basically the same.

 

Now, modifying the TM lower to be able to take M4 mags would also be a little bit of a challenge. The 556 has an extended mag well, especially compared to the normal 55x. You'd need to modify the receiver of the 552 to have that clip that would retain the M4 magazine, and while it wouldn't be that hard to imagine some ways to do that (taking inspiration from how someone did it internally with an AUG), there's limited space if you aren't just going to cut out that chunk of the receiver. Consider that the width of an M4 mag is going to be the same as the width of the top of the SIG mag, and the sides of the receiver are flush with the mag when it is inserted (those "indented" areas by the mag well). Then, there's that issue of feeding the BBs from where the M4 mag feeds to where the normal hop up chamber is. So, unless you're going to make an extended mag well, you're going to have very limited space to do that, if any at all, given that in a normal 552, the magazine basically presses right to the bottom of the hop up chamber.

 

Then, if you somehow get the receiver to take and feed the M4 mags, you may need to modify all of your M4 mags in order to fit, depending how the receiver was modified to accept the M4 mags. How it is now, there's a small bit of the back of the M4 mag that "interferes" with part of the gearbox (which I referred to when mentioning the difference in the length of the 556's receiver). You can see that the SIG mags have that notch cut out of the back, and that's because that's where the gearbox would normally interfere.

 

If you're really concerned about the width of the mags, cut off the clips that lock the mags together. You could also sand down that "ledge" thing on the mag (the one that lines up with the bottom of the receiver when the mag is inserted). The base plate might be able to be sanded as well, though, that might affect different types of mags differently. Since MAG midcaps are just two molded halves, you might sand all the way through the plastic, leaving the bottom of the mag only supported by the front and back. It might also affect other TM-type mags, since that base plate (which can be removed) is what partly holds the internals of the magazine inside the shell. Anyway, if the locking tabs are removed, the magazine becomes basically the same width as an M4 mag, and given that M4 mags don't lock together either, if you were going to make the switch to M4 mags from SIG mags, you'd be giving up that functionality anyway. So, removing the tabs should make no difference, and if anything, should solve the issue of being able to find pouches for them (given that they will now be the same witch as an M4 mag). That is the simplest solution.

 

Now, regarding the length, I can only assume you were suggesting that the ICS 50rd lowcaps have too low of a capacity and are too long, and they indeed are. Yes, there are more options for M4 midcaps with higher capacity than the MAG SIG midcaps, but maybe you'll just have to deal with that. If you were saying that the MAG midcaps were too long, I'm not sure where that argument is coming from, given that they are effectively the same size as an M4 mag. Now, if you wanted to have a short midcap, there are only 2 options I can really think of. Either find some of the less common TM 60rd short mags, or look into getting some custom made magazine internals to replace the internals of a normal short mag (i.e. make some rapid prototyped guts for the magazine with as much capacity as you can get with it still functioning properly).

 

 

I took some pictures comparing a bunch of aspects of the KA SIG 556, JG SIG 552 (which should be essentially identical to the TM), and a G&G SIG 550 in terms of receiver sizing. I couldn't get them to "fit" together completely since there are difference in the builds, so they don't actually fit together. Basically, those pictures just show incompatibilities.

 

Here are some pictures comparing some different SIG mags, just so you can see in terms of length and width how SIG mags compare to M4 mags, and how you might be able to just modify your SIG mags to be able to be compatible with the M4 mag pouches:

th_long-length.jpg th_long-width.jpg th_long-width2.jpg th_short-length.jpg th_short-base.jpg

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