Effect of Cannelure on Accuracy?

[Phil3]

Does the presence of a cannelure on a bullet affect accuracy? I am shooting an AR15 right now, and shot some inexpensive ammo through it. I expected better accuracy, and wondered if the cannelure on the PMC ammo hurt accuracy at all. I will be reloading next month, and am sure accuracy will improve, but just wondered about the cannelure.

- Phil

 

[Travelor]

When the Clinton gun grabb was going on, everone was buying like they are now. Midway had 224 bullets that were both with and without cannelures. The bullets with the cannelures shot worse that the ones without.

Just my observations.

 

[Laurie]

Phil,

a cannelure is never regarded as desirable accuracy-wise as it distorts both jacket and core to some extent or other and adds another potential source of inconsistency and/or ruining the bullet's concentricity. So, you won't find them on any out and out match bullets, and even the ex-government Radway Green plant here in the UK dropped the cannelure in its 7.62mm sniper round when it adopted a 155gn FMJBT design. They can also reduce the effective length of already short bearing surfaces on some designs - this applies particularly to 142-150gn 7.62mm NATO FMJBTs.

Having said that, many expanding sporting bullets with this feature made by Hornady amongst others shoot very well.

Cannelures are most often found on milspec FMJs. These are cheap bullets made to a price - I think Americans have a saying that goes like "...good enough for Uncle Sam"? that sums it up. You get what you pay for which is bullets produced on slack dies that would long since have been replaced by Sierra, Nosler, Berger etc. Jackets are poor quality compared to commercial match and even hunting bullets and you get imbalances through their being marginally thicker on one side than the other.

The FMJ design with its exposed core in the base often with a considerable concave area or cup is inherently poorer than an HPBT design too or sporting bullet with the jacket covering the base. It is surmised gases exiting the muzzle catch the rear end of FMJs as they overtake the bullet and induce random tipping effects.

Finally, look closely at your FMJBTs. You'll often find badly damaged tips caused by the ejection pin of the nose-form die and slight (sometimes marked) variations in the depth of the rear end of the lead core in relation to the jacket.

The surprising thing is often how well these bullets do, when they should perform really badly. A common and frustrating thing though is that a fair percentage group well, but there's usually a flyer. That's poor even for 'practice ammo' - you don't know whether it's your load, your rifle, your shooting techiques or what that gave the poor shot(s) making it difficult to learn.

Laurie

 

[fdshuster]

Laurie: It could not have been said better. I hope those who continue to insist that crimping bullets is a requirement for accuracy are paying attention. Several of us at one of my local CMP affiliated clubs purchased the Bushmaster CMP Match rifles thru the CMP program. Rifles arrived and we all began checking them out for reliability and accuracy. Several began reporting 5 shot "groups" at 100 yd, fired off a sand bag front rest averaging 4" to 6". They naturally blammed the rifle(s). We got together to sort things out and the first thing discovered was they were using the MILSURP,imported, 55gr.FMJ, crimped ammo. Without any other changes, except switching to match grade ammo, loaded with the Sierra 69 gr. HPBT #1380, groups turned into moa or slightly less. Kind of like buying a 911 Porsche and feeding it 85 octane fuel, then complaining about the lack of performance.

 

[Phil3]

Thank you. I asked because I built an AR15 with all the very best parts (Krieger barrel (1:9'), Geissele trigger, etc...), and it did not shoot as accurately as I had hoped. Best 5 shot group at 100 yards being .650" or so, shooting off a range sand bag with no rear rest. Most groups were .75". I am very new at shooting, so I am probably not helping. I was using PMC Bronze 55 grain FMJ ammo with a cannelure. I used a Sinclair bullet comparator and was shocked to see that cartridge length from bullet ogive to base of cartridge varied by .020" on just 5 rounds measured. Not quality ammo.

I won't be doing crimps on my reloads since most AR15 owners say it is not needed. Good neck tension should hold the bullet in place, not withstanding the AR15's slamming around of the round. I will check for bullet movement by withdrawing a chambered round and comparing length vs when I assembled the round.

 

[fdshuster]

phil3: Just a suggestion. If you're shooting your AR off a front sand bag rest, try placing the bag as close to the front of the receiver as you can get it without touching. M.L.McPherson did an article in "Precision Shooting" detailing the effects how important bag placement is. Out near the sling swivel was the worst, midway was better, best consistant groups were with the bag all the way to the rear. I tried it and it works, and have a copy of the article.