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Hi Graham, The handrails are 6 1/2 inches in length. I'm not sure on the normal ar15 rails but i know these will fit and are recommended by ruger AIM Sports® Stanag 4694 Carbine Length Quad Rail is rugged and dependable.
Add accessories with this durable Rail. This Stanag 4694 Carbine Length Quad Rail has a 2-Pc. "drop in" design and is compatible with most carbine length direct impingement gas systems.
More features:
Black anodized aircraft-grade aluminum
Accepts all standard Weaver® / Picatinny 1913 accessories
No gunsmithing required
Length: 6 1/2"
Weight: 12 ozs.

You should read through this 2 page thread (Especially page 2) so you can avoid the "teething" issues folks ran into with the Ruger's slightly different barrel nut. These folks found several "drop in" rails that work and many that did not. http://rugerforum.net/ruger-semi-auto/122131-ar556-what-quad-rail-fits.html

These keymods will work budsgunshop.com/catalog/product_info.php/products_id/5610/Bravo+KMRRPBLK5+BCM+KeyMod+Rail+Panel+Kit+AR-15+5.56mm+Polymer+Black

Here is a link of the ones i'm talking about sportsmansguide.com/product/index/aim-sports-stanag-4694-carbine-length-quad-rail?a=1720192

handguards sorry.