Almost every collapsible AR-15 stock sold today is made in two versions, and the only difference between them is a thirty-thousandth of an inch. Buy the wrong one and it will not go on the rifle. It is the single most common fitment mistake in AR-15 parts, and it takes about ten seconds to avoid.
What the difference actually is
The stock slides onto the receiver extension — the tube running back from the lower receiver, universally called the buffer tube. There are two diameters in circulation:
- Mil-Spec: roughly 1.14in outside diameter.
- Commercial: roughly 1.17in outside diameter.
That is about 0.03in — under a millimetre. It is far too small to eyeball, and far too large to force. A mil-spec stock will not slide over a commercial tube, and a commercial stock on a mil-spec tube rattles.
The names are a little misleading. Mil-spec tubes are made to the military dimension and are the common standard on most current builds; commercial tubes were a manufacturing convenience that stuck around. Neither is exotic — you simply need to know which one your rifle has.
How to tell which one you have
Three checks, in order of how reliable they are:
- Look at the back end of the tube. A mil-spec receiver extension ends in a straight, flat, 90-degree face. A commercial tube is usually cut with a slight slant — around five degrees — so the rear looks angled rather than square. This is the quickest visual tell.
- Measure it. Callipers across the tube where the stock rides. Around 1.14in is mil-spec, around 1.17in is commercial. If you have a stock to hand, slide it on — the right one moves freely with no play.
- Ask the rifle's maker. If the rifle came as a complete build, the specification sheet will say. Do not assume from the brand: plenty of manufacturers have used both over the years.
If you cannot check before ordering, that is worth an email rather than a guess. We would rather answer the question than process a return.
What we stock
Every collapsible AR-15 stock we carry is the mil-spec-diameter version. That is deliberate: mil-spec is what the overwhelming majority of current rifles use, and it is what the manufacturers themselves treat as the default. Our product pages say so explicitly, and where a stock is offered in both diameters we list the mil-spec one.
So the practical rule for shopping here is simple: confirm your tube is mil-spec, and everything in rifle & shotgun stocks fits. If your rifle has a commercial tube, you have two options — replace the tube with a mil-spec receiver extension, or buy a commercial-specific stock elsewhere. We do not currently stock commercial-diameter stocks or replacement receiver extensions, and it is better you know that now than after checkout.
Where this bites: some popular stocks
- Magpul CTR Carbine Stock — the friction-lock classic, mil-spec diameter.
- Magpul MOE SL and MOE SL-S — slim, light, tool-free slide-on fit.
- Magpul STR — CTR with QD sling sockets built in.
- B5 Systems Enhanced SOPMOD and B5 Bravo — the SOPMOD lineage.
- BCM Gunfighter Stock Mod 0 — hard-use, minimal snag.
- Reptilia RECC·E Carbine Stock — adjustable comb on a carbine tube.
The stocks where the question does not arise
Not every stock rides on the outside of the tube in the same way, and a few change the conversation:
- Precision-adjustable stocks. The Magpul PRS Lite and PRS GEN3 fit AR-15 and AR-10 pattern rifles on a mil-spec carbine receiver extension — commercial tubes will not take them.
- Complete stock kits. The BCM Gunfighter Stock Kit Mod 0 and Mod 1 kit include the receiver extension, so the diameter question is answered in the box. If you are unsure what your rifle has and want it settled, a kit is the clean route.
- Chassis systems. A Magpul Pro 700L chassis replaces the whole stock assembly on a Remington 700 and has nothing to do with AR buffer tubes at all.
- Adapters. The Reptilia LINK adapter puts a Magpul Zhukov stock on a CZ Scorpion EVO — again, a different interface entirely.
A note for straight-pull rifles
UK straight-pull AR-15s use standard AR-15/M4 pattern lowers, so the stock fits exactly as it would on any other AR-pattern rifle — the buffer tube question is the same question, with the same answer. What changes is what you should prioritise: without a self-loading cycle to work around, comb height and length of pull matter more and recoil management matters less. Our straight-pull AR-15 parts guide goes into it properly.
Once the diameter is settled
Diameter is a yes/no gate, not a buying decision. Once you know your tube is mil-spec, the real choices are length of pull, cheek height for the optic you are running, and whether you want a QD sling socket or a storage compartment. That is covered in how to choose an AR-15 stock, and if the optic is driving your comb height, read the optic mount height guide alongside it.
Browse rifle & shotgun stocks, or see what fits the budget in AR-15 upgrades under £100. £5 tracked UK delivery in 7–10 business days.
