What size is the pistol grip screw on an AR15?
1/4-28 if I remember correctly.
1/4-28 is correct.
Can anyone remember how long?
I’m not sure how they measure screw length, but the thread portion on the spare I have here is 1" long.
Screws measure from the back of the head. 1 inch is correct.
The handgrip screw is 1/4-28x1" cap head and should correctly be torqued to 35 inlbs. The current drawing pack indicates that the lock washer is an ouside tooth design, but this is only effective at locking the screw if the older slot head is used. The lock washer should be substituted away from specifications for an inside tooth design if it is to be used effectively with a cap head screw. A 1/4-28x3/4 screw may be required for some lowers as the thread may not extend the full length of the hole.
Bill Alexander
Where to buy?
I have an allen head one and washer if you want (free), let me know where to send it.
Thanks gotm4 but I am trying to replace the allen socket head screw that I have with a slot head screw so I can remove it with a common screwdriver. Do any of the site’s vendors have them on their web sites?
Dude,
Go to Home Crepot and get a 1/4-28x1 slotted-head cap screw + lock washer and be done with it.
Dude 
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Maybe Home Depot carries fine thread machine screws. Lowes does not - all they have is 1/4-20. There is not a Home Depot in the town where I work.
Brownells has them. (DPMS brand) www.brownells.com
#231-000-134 Pistol Grip Screw $1.50
#231-000-135 Pistol Grip Lock Washer $0.71