AR-15 Barrel Question

[Asa Yam]

you definitely want at least a 1:9 twist. while you can cycle 55 grain bullets with a 1:12, they are a rarity (the twist rate) today on your garden variety AR.

A little history on the AR-15: The first rifles (Late 1950s/early 1960s) were equipped with 14 twist barrels. This was changed to a 12 twist when tests found 55 grain FMJBTs were keyholing on thrgets in cold weather (Alaskan winter). Faster twists appeared in the mid- to late- 1980s when the M16 was revised to fire 62 grain FMJBT and the associated (longer) 64 grain tracer round.

Not sure where I read this, but gas tubes can be made from stainless steel brake line tube. This piece of knowledge is handy if you are planning on building a longer than normal gas tube. On a 24 to 26" long barrel, relocating the port further down the barrel is a good thing, due to a feature known as "dwell time". In a rifle, dwell time is the amount of time the gas system remains under pressure. Depending on pressure and time, different things can happen:

• Low pressure, insufficient time: Rifle fails to extract or eject.
• Low pressure, sufficient time: Rifle may extract, but fails to eject. (Short cycle.)
• Low pressure, excessive time: Rifle may function properly, or it may short cycle.
• Adequate pressure, insufficient time: Rifle may cycle properly, or it may short cycle.
• Adequate pressure, sufficient time: Rifle cycles properly.
• Adequate pressure, excessive time: Rifle cycles, but extraction is harder than normal. (Extractor marks on case rim.) Cyclic rate increases as a result.
• Excessive pressure, insufficient time: Rifle may cycle, or it may short cycle.
• Excessive pressure, sufficient time: Rifle cycles, with the occasional extractor mark on the case rim.
• Excessive pressure, excessive time: Rifle fires, but cases remain in chamber (extractor tears through rim), OR; rifle fires and ejects, but fails to pick up another round, OR; rifle fires, ejects, and feeds normally, but rims on ejected cases are bent.

Notice that out of nine possible outcomes, only two produce acceptable outcomes?

Fixes for the above:

• Pressure. Use the correct powder. Semiauto full length ARs generally do best with powders in burn rate between H-335 and Varget. Actually, that's not a bad place to be, as lots of good powders are in that range. (Personal favorite is IMR or H-4895.) Gas port diameter is (at best) a partial fix for pressure, as is gas tube length.
• Time. On an AR, dwell time is based on how long the bullet remains in the barrel after it passes the gas tube. A longer barrel requires the tube be placed further down to maintain an adequate dwell time. However, the additional distance will not be more than 2" on a 26" long tube, as gas port pressure will be lower.

Med, if you haven't done so already, I recommend you buy the following book: "The Complete Guide to AR-15 Accuracy". This is because the book has information on building accurized ARs (including those with longer barrels). Used to be you could buy copies from the author himself - Derrick Martin. However, his website seems to be broken at present. Copies are available from a number of sources, including Amazon.com: http://www.amazon.com/Complete-Guide-AR-15-Accuracy/dp/0967094852
Hope this helps.

Asa

 

[ultramagmed]

Thanks again to you all!

I got the specs I needed (thanks Mike), plus I got a lot of information I didn't expect to get that will help me after my project is done. I know this project may be a less than ideal platform since the barrel is a factory Remington 22-250 barrel, but I thought what the hell, not gonna use it for anything else, may as well learn something with it. I'll report back when I get it done.

Thanks guys, I appreciate all your help.

Med.

 

[msalm]

I've done that once before with a 22 cf take-off barrel...didn't shoot worth a hoot, in fact I remember 55 or 50gr ballistic tips keyholing in that barrel...hope you have better luck!