BTO giving me fits!

[david walter]

I have an Aero M5 in 308 with and Aero barrel. It has an adjustable gas block, and I’ve been trying to measure BTO to avoid jamming rounds into the lands, which I’ve heard is bad with semiauto rifles.

using a Hornady OAL gauge and Hornady electronic calipers, I get:

155 Scenar - BTO = 2.233 inches average of five measurements

150 Hornady STS - BTO = 2.258 inches average of five measurements.

I thought BTO was a constant across bullets?

Or am I not understanding to measuring right?

 

[FCS]

What?!
None of your ramble / rant / vent resembles a logical progression of what is happening.

I have zero understanding of what you did, what you hope to do or how you did your measurements.

No, BTO is usually less variation BTO than base to tip on average. A “constant” as in exactly the same from one bullet to each in a given brand, one single lot #....uhhhhh, LMFAO, no it will not be exactly the same.

Willing to bet SAAMI chamber in Aero bbl.
If you are running these out of magazine, you’ll have jump and probably more than 0.050” of jump as 2.830” is pretty much going to be your max OAL. KAC / LaRue mags being exception, allowing 2.860” OAL.

BTO is not going to be exactly same with different bullets as the comparator is measuring at a diameter other than 0.308, probably more like 0.3055 or so.
Ogive profile between 0.308 and 0.3055 on two different bullets (ogive profiles) can be quite a bit different. So pretty easy for there to be 0.025” difference due to ogive profile from 0.308” diameter (where bullet hits lands) and 0.3055” diameter (where comparator lands on bullet).

What is OAL for 155 Scenar @ 2.233” BTO?
What is OAL for 150 Hornady @ 2.258” BTO?
Ammo going to be run out of magazine or single loaded?

 

[david walter]

Ammo out of a magazine.

your explainarion of the differences is just what I needed. Thanks.

any of these loads will be away from the lands when loaded in a MagPul mag, so I think I’m good.

 

[nn8734]

I have an Aero M5 in 308 with and Aero barrel. It has an adjustable gas block, and I’ve been trying to measure BTO to avoid jamming rounds into the lands, which I’ve heard is bad with semiauto rifles.

using a Hornady OAL gauge and Hornady electronic calipers, I get:

155 Scenar - BTO = 2.233 inches average of five measurements

150 Hornady STS - BTO = 2.258 inches average of five measurements.

I thought BTO was a constant across bullets?

Or am I not understanding to measuring right?

How do you know your jam length for your rifle and those two above mentioned bullets?

Did you use the Hornady case length gauge with modified case to measure and determine the BTO when jammed? Did you Ogive sort your bullets before taking any such measurements?

I would be surprised if you were able to get anywhere near the lands and still fit those rounds in the magazine.

The average BTO isn’t as important as the range of measurements for your sample. If your jump varies significantly enough from round to round it will show up on your targets, esp beyond 500 yards.

What was the total variance in your measured sample? If it’s less than .005, it’s likely not going to make any difference.