AOL and velocity

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AOL and velocity

Postby Cutright » Mon Jan 25, 2016 5:38 pm

On my bolt guns I was surprised how big of a difference the length of the bullet made on accuracy. In the AR you are limited. Does the length still affect accuracy or not so much? How about velocity? I have a bolt gun that no matter what if you can get the bullet to go 2800fps it will be accurate. Does my 450bm have a "favorite" speed? Sorry for all the stupid questions!
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Re: AOL and velocity

Postby Texas Sheepdawg » Tue Mar 29, 2016 1:19 am

You probably need to measure your free bore. Then when you're reloading you'll know exactly what to set your COAL to.
Yes, your COAL will influence velocities as well as accuracy. It does seem that some rifles have a "pet round" that performs best only out of that particular rifle. That is why it's so important to measure your chamber and freebore.
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Re: AOL and velocity

Postby Hoot » Tue Mar 29, 2016 7:21 am

WRT semantics. Since when is "overall" two words? ;)

Dawg is right. Get a Hornady Overall Length Gauge and 450b case for it. The favorite speed changes from bullet to bullet. However, over time, my original 1:24, 20", factory barrel with Ross brake (yes that matters) loved velocities in the 2200-2250 range. Hard to shoot a bad group in that speed range. Again, there will be other accuracy nodes at different velocities, but they are bullet dependent. For the 225gr FTX, I have personally seen three velocity accuracy nodes and that is consistent with the physics of a barrel such as documented on Varmint Al's web page.

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Re: AOL and velocity

Postby Cutright » Thu Mar 31, 2016 8:51 pm

I have the guage and the case. However the length of the case where it meets the lands is far longer than the magazine will allow, so I've just been loading at factory length
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Re: AOL and velocity

Postby Hoot » Fri Apr 01, 2016 6:47 am

Cutright wrote:I have the gauge and the case. However the (edited)COL when it meets the lands is far longer than the magazine will allow, so I've just been loading at factory length


You have just rediscovered the downside of the AR15 platform. Different bullets with their individual ogives will engage the lands at different COL. I should clarify that "engaging the lands" is a somewhat misnomer. Many bullets engage the taper of the throat, at the end of the leade, before actually engaging the lands, which begins shortly beyond that point. I'll give you a rare example in the 450b. The Hornady 300gr XTP Mag does not have a ballistic tip that extends its length. As such, in my chamber it engaged the lands with the COL only being 2.22. I suspect that would be applicable to any non-ballistic tip hollow point, that is long such as occurs in the 300 grainers, but I only checked it using the XTP Mag. Additionally, due to its dense nature, the short, 230gr, round nose FMJ engages the throat at a COL that places it totally outside the case. IE it is clear of the case before it starts obturating to fit the bore. In my mind's eye, I believe it is really important for the leading edge of the bullet to be supported by the throat before it is no longer supported in the rear by the case. Otherwise, it can be slightly misaligned when it enters and exits the bore. It may be why I have poor luck with groups when shooting the 230gr round nose fmj.

There is however another alternative to being limited so much by the inside dimension of the magazine. Back in 2011, I wrote up a how-to thread, for modding a magazine to allow for COLs up to or slightly longer than 2.40. Amazingly, the thread and images are still intact over at the Olympic Arms website. Here's the Link to it.

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EDIT: I must apologize because in revisiting the article referenced above, some of the images have fallen victim to Tiny Pic hosting expiration policies. The missing images would have shown my method for accurately slitting the front of the mag. cbass16 (original idea source) slit his free-hand and although it was not cosmetically as nice as my result, it still worked fine.

2nd EDIT: I found the backup images from that link. If anyone is interested in reproducing my work, let me know and I can send them to you. In the mean time, I'll try editing that old article over at oa2.org.
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Re: AOL and velocity

Postby Texas Sheepdawg » Fri Apr 01, 2016 9:50 am

And that's the reason why I've seated this particular load "out" a bit.
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Re: AOL and velocity

Postby Al in Mi » Fri Apr 01, 2016 10:31 am

Hoot, send me those pics, might have a need for this 45 project
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Re: AOL and velocity

Postby plant_one » Fri Apr 01, 2016 12:15 pm

Texas Sheepdawg wrote:And that's the reason why I've seated this particular load "out" a bit.
Image



wow. how much (or how little!) do you actually have left in the case on that one! yikes! :shock:
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Re: AOL and velocity

Postby Texas Sheepdawg » Fri Apr 01, 2016 3:03 pm

plant_one wrote:
Texas Sheepdawg wrote:And that's the reason why I've seated this particular load "out" a bit.
Image



wow. how much (or how little!) do you actually have left in the case on that one! yikes! :shock:


Here's the story.
viewtopic.php?f=5&t=8633&start=50
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Re: AOL and velocity

Postby plant_one » Fri Apr 01, 2016 9:56 pm

hot diggity. thats something else.

so if i read right, your seating depth is ~ ½ bullet diamater? or about .225? seems reasonable - that's enough bullet in that case to get a good grip on her :)
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