OBT on an AR15 question:

[fatelvis]

When figuring the OBT on a AR15 barrel, what effect does the gas block and gas tube have on the calculations? Is it something that can be estimated, or just has to be tried in the field? Obviously, it will have an effect on barrel harmonics. Thanks

 

[marazzi59]

What's OBT?

 

[Herzo]

Optimal barrel time? Probably insignificant in that the bleedoff is within microseconds of the bullet exiting the barrel. In terms or harmonics, again the gas block is a minimal weight addition. I have found the calculation by QuickLoad to be close enough to be useful. The calculation is based on the length of the barrel with no consideration of the mass.

 

[jelenko]

I asked that very question in the discord chat room for GRT.
The answer was to not specifically address it. Just use the observed velocities to have GRT tune the powder parameters to the rifle.

 

[DLT]

What's OBT?

It’s over my head for sure

 

[fatelvis]

It’s over my head for sure

It’s really neat, once you look into it. There is a chart where you look at the length of your barrel, and there are several nodes that will be accurate, depending on the speed of your bullet. It lists five different exact milliseconds, that represent the accurate nodes. You can plug in all the information about your load into quick load or Gordon‘s reloading tool, and it will give you the milliseconds the bullet will be in your particular barrel. You then use that information to look it up in the chart. Somebody, with a much bigger brain than me, came up with it! Lol

 

[jelenko]

It’s really neat, once you look into it. There is a chart where you look at the length of your barrel, and there are several nodes that will be accurate, depending on the speed of your bullet. It lists five different exact milliseconds, that represent the accurate nodes. You can plug in all the information about your load into quick load or Gordon‘s reloading tool, and it will give you the milliseconds the bullet will be in your particular barrel. You then use that information to look it up in the chart. Somebody, with a much bigger brain than me, came up with it! Lol

FYI/caution. The guy who's being doing 'education' for GRT says that about 40% of the nodes that GRT predicts are incorrect.
From my own experience, even the nodes that are 'correct', probably aren't as well tuned as benchrest competitors need.
So, it depends on the level accuracy you need as to how useful GRT is in predicting accuracy nodes.

When using GRT, it's important to record the velocity of 7-10 rounds [using a mid range load]. GRT uses the velocities to tune a couple powder parameters to more accurately predict a node for your rifle.

 

[fatelvis]

FYI/caution. The guy who's being doing 'education' for GRT says that about 40% of the nodes that GRT predicts are incorrect.
From my own experience, even the nodes that are 'correct', probably aren't as well tuned as benchrest competitors need.
So, it depends on the level accuracy you need as to how useful GRT is in predicting accuracy nodes.

When using GRT, it's important to record the velocity of 7-10 rounds [using a mid range load]. GRT uses the velocities to tune a couple powder parameters to more accurately predict a node for your rifle.

I’m using quick load. Is that any more accurate?

 

[DocBII]

Elvis,

Similar thread.

HTH,
DocBII

 

[Dimner]

I’m using quick load. Is that any more accurate?

I have never been able to get the full OBT to work on any of my AR15s. Even my match grade free floated barrel. I think the AR15 system introduces too many other variables.

What I mean is, the calculation (even if you manually calculate it) of the node you should expect good results... it never meshed up to the nodes that I got from the formulas.

Covid was a loooong year, and so I did alot of testing on this and other AR15 things. Spreadsheets, half nodes, node deltas, measuring barrel lengths down to the 1/8 of an inch, etc.

What I did find out was that there was a barrel timing that was the most accurate. for a specific barrel So in QL after i adjusted for the powder burn rate using my chronograph results... I would take the barrel time from my best and repeatable results... this result would be usable to apply to other bullets. By usable, I mean, with in like 0.2 or 0.02ms (hard to remember). I could use that barrel time to get very close to the best performance from other bullets. So I was using 55gr Hornady SPs. That barrel time got me close on loads with other 55-62ish grain bullets. I tried many different Hornady and sierras. I never tried Barnes. I will add that it never worked with Speer 55grs. Those things I coudlnt get to shoot worth a damn at any barrel time.

I'm guessing that what I found (other than the speers) had to do with bearing length rather than the actual weight. I was kind of burnt out of the project when I moved to 70gr bullets and tried to figure out the relationship of the best barrel time to what I found at 55gr.

Anyway, hope some of that helps.

 

[jelenko]

I’m using quick load. Is that any more accurate?

I don't know. Haven't used Quickload.

That said, I like that GRT has a built in OBT calculator that automatically adjusts the powder parameters based on the observed velocities. I'm thinking the expert behind GRT would do a better job of that than I would and, hence, would be more 'accurate'.

 

[jelenko]

barrel timing that was the most accurate. for a specific barrel

I think this is what GRT does using the observed velocities from a specific barrel/powder/bullet//primer/seating depth.

 

[Dimner]

I think this is what GRT does using the observed velocities from a specific barrel/powder/bullet//primer/seating depth.

They both get the same results.

In QL, you get a barrel time in MS and put it in the OBT formula in a spreadsheet. It takes in account the barrel time and length.

In GRT, they use the same formula, and do it for you.

 

[fatelvis]

Ahh I see. I’m starting to hear more and more people using Gordon’s. I am tempted to try it, but I feel like I have invested so much money in quick load, it would be a waste to leave it. Lol.

 

[Dimner]

Ahh I see. I’m starting to hear more and more people using Gordon’s. I am tempted to try it, but I feel like I have invested so much money in quick load, it would be a waste to leave it. Lol.

personally i like QL alot better. Maybe it's because I have used it for years.

 

[fatelvis]

personally i like QL alot better. Maybe it's because I have used it for years.

I know I’m going off subject, a little, but hey, it’s my thread, right?! Lol
Could you quickly point out the pros and cons of each? I’ve never seen a comparison of the two.

 

[Dimner]

QL has more cartridge choices.

GRT has a much easier interface to be able to track shots and work on your load as a 'whole project'

QL's interface is as old as window XP, or at least it seems that way.

In GRT I often make errors when I look at load increments and forget that I'm looking at a couple grains less than the load weight I put in.

QL has better velocity and burn rate estimates than I have found in GRT. This is for the cartridges that I have used. GRT seems to be all over the map. What I mean by this is: If you have a cartridge and a powder you want to try with a bullet. QL estimate will probably be a little fast as compared to what you observe on your Chrono. I'm okay with that as it means I won't be near max pressure. I'm okay with QL estimating about 100fps faster than what i observe.

GRT on the other hand is either reeeeallly fast on the estimates or too low by 100fps or more! So some loads I observe the velocity the be much slower than the GRT estimate, but then I change to a different powder, now my chrono readings are 100-200fps faster than the GRT estimate. Which means it could be in the high pressure zone. This 2nd example is kind of rare, but still not at all a good thing to deal with.

There are other benefits to GRT. Lately I have been trying to use GRT first, but then I end up in QL. Old tricks, dog and stuff.

I had never used either, I would use GRT, learn it, and never have 2nd thoughts.

 

[fatelvis]

Thanks for your insight Dimner. Many good points of interest . Although I don’t like your conclusion! Lol

 

[karrarararararar]

I wouldn't worry about the gas block. Ever notice there's no option for barrel profile?