TENSION SCREW
Frequently Asked Questions — SOTM4 & SOTM5 Billet Lower Receivers
Two receivers. One rifle.
// The Short Answer
An AR is two halves joined by two pins. Pins in holes need clearance to work.
Your upper and lower receivers are held together by a pivot pin at the front and a takedown pin at the rear. Those pins have to slide in and out by hand, which means their holes are cut with clearance — and clearance is play. Stack that against normal manufacturing tolerance on both receivers and most AR-pattern rifles have a small amount of movement between the halves. You feel it as a rattle when you shake the rifle, or as a faint shift when you pull the stock into your shoulder. The tension screw is a small nylon-tipped set screw threaded into the lower receiver that bears upward against the upper, taking up that clearance and holding the two halves together as a single unit. It is machined into the SOTM4 and SOTM5 billet lowers as a designed-in feature, so no aftermarket wedge or rubber insert is needed. On an all-SOT billet pairing there is very little for it to take up — our uppers and lowers are held tight enough to each other that the screw is not solving a problem we shipped you. Where it earns its place is the moment you put a different upper on the lower.
// What It Fixes, And What It Doesn’t
We want to be careful here, because this feature is routinely oversold.
Your barrel is mounted to the upper receiver. Your optic is mounted to the upper receiver’s rail. Both ends of the aiming system live on the same half of the rifle, which means play between the upper and the lower does not move your point of impact relative to your sights. Anyone telling you a tension screw will shrink your groups is telling you something the geometry does not support, and we would rather say so than sell it.
What it does change is everything that crosses the joint. Your trigger, hammer, and fire control group are in the lower; the bolt carrier and firing pin they act on are in the upper. Your hand, your shoulder, and your stock are on the lower; the barrel that has to return to the same place is on the upper. Removing the play makes those relationships consistent instead of variable, and it stops two aluminum lugs from working against each other under recoil, which is how takedown lugs get peened over a long service life.
There is a second reason it’s there, and on our own rifles it’s the main one. Our billet uppers and billet lowers are machined to tolerances tight enough that a matched SOT pair has essentially no slop to remove — the screw has little work to do on a rifle we built end to end. But a lower receiver outlives any single upper. Shooters swap uppers between calibers, barrel lengths, and configurations, and the upper going on is often another manufacturer’s — frequently a forged receiver, which is not held to billet tolerances and was never intended to be. The tension screw is what lets a SOT lower take any of them and still close up tight, instead of being a receiver that only feels right with its original partner.
The honest summary: this is a feel, consistency, and longevity feature, not a group-size feature. It is also the difference between a rifle that feels like one object in your hands and one that feels like two parts pinned together. On a precision rifle, where every other variable has already been engineered out, that matters more than it sounds like it should. Confidence in the rifle is part of the equipment.
| The feature | As built at Second of Ten |
|---|---|
| Type | Nylon-tipped set screw, threaded into the lower receiver |
| Bears against | The upper receiver — takes up upper/lower clearance |
| Contact material | Nylon — will not mar, gouge, or dent the upper receiver |
| Adjustment | User-adjustable by hex key, in the field, without tools beyond the key |
| Retention | The nylon element resists backing out under recoil and vibration |
| Fitted to | SOTM4 billet lower receiver · SOTM5 billet lower receiver |
| Primary use case | Mixing non-SOT uppers — including forged receivers — onto a SOT billet lower |
| Requires | Nothing else — no aftermarket wedge, rubber insert, or shim |
| Serviceable | Yes — the screw is a replaceable item |
| Effect on takedown | None when correctly set — pins operate normally |
// Questions We Get Asked
Why is the tip nylon instead of just steel?
This is the part of the design that actually took thought. A bare steel set screw would do the tensioning job and create three new problems:
- It would mar the upper. Steel bearing on anodized aluminum under preload digs in. You’d get a witness mark, then a dimple, and eventually a divot the screw settles into — at which point your adjustment is gone and you’ve damaged a receiver.
- Nylon has a little give. It takes up the gap and holds preload without concentrating the whole load on one hard point. The contact spreads instead of digging.
- Nylon is its own thread locker. The same property that makes nylon-element lock nuts work keeps this screw from walking out under recoil, with no compound in the threads to reapply.
If your receivers are that tight, why does the rifle need a tension screw at all?
On a rifle we built end to end, it mostly doesn’t — and we’d rather say that than pretend we sold you a fix for a problem we created. Our billet uppers and billet lowers are held to each other tightly enough that a matched SOT pair has very little play to remove. The screw is there for what happens next:
- Lowers outlive uppers. A lower receiver is the serialized part you keep. Uppers get swapped for different calibers, barrel lengths, and roles over the years.
- The next upper often isn’t ours. Shooters mix and match, and a SOT lower should accept another manufacturer’s upper without feeling like a compromise.
- Forged uppers aren’t held to billet tolerances. That isn’t a criticism — forged receivers are made to a different standard for good reasons, and we build forged uppers ourselves. It does mean the fit varies more, and the screw is what absorbs that variation.
Will it make my rifle shoot tighter groups?
Not directly, and we won’t claim it will. The barrel and the optic are both on the upper receiver, so play between the halves doesn’t change where the bullet lands relative to your sights. If a rifle you own got noticeably more accurate after adding a tensioning device, something else was going on — usually a loose optic mount or a handguard problem.
Then what’s the real benefit?
Consistency across the joint and durability of the receivers. Your fire control group acts on parts in the other half of the rifle, so removing the play makes trigger and hammer geometry repeatable. Your shooting position loads the lower while the barrel lives on the upper. And two lugs that can’t move against each other don’t peen over thousands of rounds. Add the fact that the rifle stops rattling, which shooters notice every time they pick it up.
How do I adjust it?
With the rifle assembled and unloaded, turn the screw in with a hex key until the play between the receivers just disappears, then stop. Grip the rifle and try to move the halves against each other — when that movement is gone, the screw is set. It’s a feel adjustment, and it takes about thirty seconds.
Can I overtighten it?
You can, and you shouldn’t. Cranking past the point where play disappears starts prying the receivers apart at the rear, which can make the takedown pin stiff to remove and puts load on the lugs the design never asked for. More tension is not more better. Snug and stop.
How is this better than a rubber wedge?
A drop-in wedge is one fixed size for every receiver pairing, and it can’t be dialed to the amount of play your rifle actually has. Rubber also compresses over time and stiffens or softens with temperature. A threaded screw is adjustable to your rifle, re-adjustable years later, and doesn’t rely on a consumable block staying the same size.
Does it interfere with field stripping?
No, when it’s set correctly. The takedown and pivot pins operate normally and the upper hinges open the way it always has. If separating the halves feels noticeably harder than it should, the screw is in too far — back it off a quarter turn at a time until it isn’t.
Will the nylon wear out?
Eventually it may take a set, which shows up as a little play returning after a long time in service. The fix is a quarter turn in, and if it ever needs more than that, the screw is an inexpensive replaceable part. That’s the point of putting the wearing surface on a component that costs almost nothing rather than on your receiver.
Do I need to readjust it if I change uppers?
Check it, yes — and this is exactly the case the screw exists for. Every upper sits slightly differently, and the further the new upper is from our own tolerances, the more there is to take up. Another maker’s billet upper may need a fraction of a turn. A forged upper may need noticeably more. Either way it’s the same thirty-second feel adjustment, which is the advantage of an adjustable screw over a fixed insert — it follows whatever you put on the lower.
Is it on both the SOTM4 and SOTM5 lowers?
Yes — the small-frame SOTM4 billet lower and the large-frame SOTM5 billet lower both carry it as standard. It’s machined into the receiver, not added afterward, which is one of the features that requires billet manufacturing in the first place.
Is a rattling rifle actually a problem?
Mechanically, a mil-spec AR with normal upper/lower play runs and shoots exactly as designed — the rattle is a tolerance artifact, not a defect. We’re not going to pretend otherwise. But a rifle sold as a precision instrument shouldn’t feel loose in the hands of the person who paid for it, and the fix costs a set screw.
// What The Tension Screw Does Not Do
One small feature, honestly scoped.
- It doesn’t change your point of impact. Barrel and optic are both on the upper. This is worth repeating because it is the claim most often made for this feature and it isn’t true.
- It doesn’t repair worn or out-of-spec receivers. A tension screw takes up normal manufacturing clearance. It is not a fix for elongated pin holes or damaged lugs.
- It doesn’t substitute for correct pin fit. Pivot and takedown pins still have to be right. The screw addresses what’s left after they are.
- It isn’t a torque specification. There is no number to look up. Snug until the play stops, and no further.
- It doesn’t address handguard flex or the barrel joint. Those are separate problems with separate answers, covered in their own sheets.
// What You Get
Your rifle leaves here with the tension screw already set to the upper it shipped with. You don’t have to do anything on day one. Keep the hex key that fits it with your rifle — the day you put a different upper on that lower, ours or anyone else’s, that key is the whole procedure. If a little play shows up after a few thousand rounds, same answer: a quarter turn and thirty seconds. If you’d rather we look at it, send it in; that’s what the build record is for.
No rattle. No wedge. No overclaim.