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Bite The Bullet

Will it fit? AGM M4 Compatibility review (2009-06-17)

Untitled Document

It has been asked various times on what parts are compatible with what GBB rifles. We have decided to put this to the test to see if the claims of compatibility are true. This test of compatibility will be based around the AGM version if it would be able to use other branded parts and still work.

The AGM rifle in question about to be tested

The parameters for testing are as follows.

Gas ? guarder green gas
Ammo ? KSC perfect .20g
Ambient temperature ? 21⁰C

There?s what the guns in question chrono at straight out of the box.

G&P Stock

AGM Stock

Western Arms Stock



A test was done to see if the complete upper and lower receivers would be compatible for a straight swap, here?s what we found.

Upper and lower receivers are NOT compatible to do a direct swap between G&P and AGM.

The AGM have a wider tab on the front and back of the upper receiver that does not fit the G&P lower receiver although with a push, the upper front receiver tab on the AGM can fit the G&P lower. Compared to the G&P upper receiver has too wide a rear tab to fit the AGM lower receiver. Similarly to the WA lower when the AGM upper is used, both the tabs are too large to fit into the lower receiver into the locking position so the AGM upper cannot be secured into the other 2 brands of GBB M4?s.

The lower receiver from the AGM however, due to its larger width available to accommodate their larger receiver tabs, can accommodate other branded upper receivers but it will not be secure and the mechanisms will not work as efficient as intended due to the slight misalignments. This doesn?t mean it cannot function but it will impair its performance.

Slight misalignments and dimensional differences stop the receiver from being compatible with other brands.


AGM upper front 14.20mm G&P upper front 12.62mm WA upper front 14.02mm
AGM upper back 11.00mm G&P upper back 10.48mm WA upper back 9.76mm
AGM lower front 14.26mm G&P lower front 13.00mm WA lower front 14.12mm
AGM lower back 12.20mm G&P lower back 10.92mm WA lower back 9.86mm

The G&P can accept AGM bolts with a slight wobble and fire with a slight decrease of fps whereas the AGM accepts the G&P bolt but no rounds are fired out of the barrel with the bolt cycling. We then took all the bolts out to find out what was causing this problem and pinpoint the cause of this.

This is the measurements we found:
AGM bolt ? 1.08mm recess
G&P bolt ? 0.6mm recess
WA bolt ? 1.08mm recess

The nozzle and hop-up dimensions stop it being completely compatible with some aftermarket bolts.

The nozzle on the G&P bolt protrudes forward compared to the AGM bolt, the BB is pushed too far forward to activate the rocket valve so the BB ends up literally rolling out of the end of the barrel even when the bolt cycles without a problem.

Magazines however, are also not compatible between the G&P and the AGM because of the valve placement and dimensions are different. Although they look exact on the surface, the release valve on the AGM is positioned slightly lower than the G&P mag in comparison with the G&P mag?s release valve angled higher when compared with the AGM mag. So when the AGM mag is used in the G&P, the gun can chamber a round from the AGM mag, but it cannot fire due to the firing pin not hitting the correct place to release the gas to propel the round at all.

When the G&P mag is used in the AGM, the dimensional difference just wouldn?t allow the mag to even be inserted to the gun at all. This is partly due to the additional ribbed magwell on the AGM as well as the slightly smaller magwell on it.

The WA mag, however, works in the AGM rifle and chambers a round. However, the mag catch does not work when the mag is empty. On the contrary when the AGM mag is used on the WA, the hammer on the WA does not hit as hard as the G&P or the AGM since it?s designed for use with 134a, making the WA shoot weaker than the market stall?s budget springer pistols. Please remember, it is NOT recommended to use any propellants other than 134a on stock WA parts. This was only done to show how the parts perform under the same conditions.

G&P with AGM bolt (fps) AGM with G&P bolt (fps) WA with AGM bolt (fps)

To summarise, even though the AGM is said to be WA compatible, it is not fully compatible due to the size and orientation differences between the brands. The mags cannot be switched between the AGM and the G&P models as mentioned earlier due to the placement on the release valve and although the WA and AGM can share mags (with a tight fit on the AGM due to its ribbed mag well) the WA mag is only designed for 134a propellants and therefore will not cycle the gun properly since the bolt and spring is heavier on the AGM.

If you wish to purchase or acquire parts from a different brand to use with your GBB rifle, it is highly recommended that you find out from other sources such as community forums to confirm the compatibility on specific parts just to be sure it will not be an issue.