Question regarding powder burning rates

[JBomb]

Hi guys,

I was hoping someone could clue me in regarding powder burn rates. I'm reloading for a 308 AR with a 18" barrel, I'm trying to find a good load using the Hornady 178 BTHP. I have tried using Varget although I'm getting pressure signs (ejector marks) at about 40.5 grains. I'm not happy with the velocity at only 2350 fps...

I thought about using IMR 8208 xbr, but I see its a faster burning powder than Varget. With that said, will I just see pressure signs faster than Varget with no increase in velocity? Should, I go to a slower burning powder?

Kind regards,
Jared

 

[mac37]

Re: Question regarding powder burning rates

What are you expecting out of an 18" gas gun? Sierra manual suggests IMR 6064 as a go to powder for semi-auto .308's.

 

[JBomb]

Re: Question regarding powder burning rates

Mac, I was hoping for around 2450- 2500....which is about the same as I get from 175 FGMM.

Kind regards,

Jared

 

[QRE23]

Re: Question regarding powder burning rates

Something to keep in mind is that those burn rate charts and lists aren't etched in stone. I used to think powder rate was a constant but after talking to a Hodgdon Tech a while back he explained to me that burn rates are variable and often change depending on the case they are fired in. The chart is a general somewhat variable guide.

 

[sawgunner2001]

Re: Question regarding powder burning rates

I'm working up a load for a 20" gasser as we speak. A brief summary of my results follows:

H4895

42.0gr = 2562fps The primer pockets were still tight (and this is an accuracy node)
42.5gr = 2600fps The pockets began loosening up (I didn't know this till I tried to seat another primer...so I kept going up cause the primers still had nice, rounded edges)
43.0gr = 2635fps
43.5gr = 2658fps
44.0gr = 2675fps (another accuracy node)
44.5gr = 2703fps (also accurate, but a little more vertical with slightly higher SD)

Varget

43.0gr = 2516fps primer pockets still tight
43.5gr = 2546fps primer pockets slightly loose, but still snug
44.0gr = 2585fps loose pockets
44.5gr = 2600fps
45.0gr = 2654fps


I really wanted to find an accurate load with Varget (I've got about 6 lbs of it...and only 1 lb of H4895), but H4895 did quite a bit better.

I don't recall where I started seeing ejector swipe, but it was early. I'd read about this in several places and found that guessing chamber pressure in a gasser is difficult since you have two factors in play, those being chamber pressure during ignition and chamber pressure during extraction.

The best way I've found to judge chamber pressure during ignition is by decapping the spent primer and inserting another. This sucks because, well, its a pain in the ass. I've read about measuring case head expansion, and did that on my fired casings. I found it to be unreliable since my measurements were all over the place (some lower loads measuring more expansion than higher ones, etc).

It was nice when I was loading for a bolt gun and had tangible and reliable signs to look for such as ejector swipe and sticky bolt lift.

FWIW, I think I'm gonna settle on the 42.0gr of H4895 load. It's fairly accurate without excessive pressure in my rifle. It's too bad the 44.0gr H4895 load is too hot, cause five rounds were covered up by a nickel.

Anyways, this has gotten to be a bit long winded so I'll let others comment on this and your quest for a gas gun load.

Edited to Add:

The above data was gathered using Winchester brass, 178gr A-Max bullets, and CCI BR2 primers.

 

[Iamironman]

Re: Question regarding powder burning rates

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: JBomb</div><div class="ubbcode-body">Mac, I was hoping for around 2450- 2500....which is about the same as I get from 175 FGMM.

Kind regards,

Jared
</div></div>


Go to this site

Hodgdon

Then type in your data, then just browse through the powders they have already tried and pick one rather than guess.

 

[JBomb]

Re: Question regarding powder burning rates

Thanks for the responses guys!

I guess I'm more curious to the factors that cause excessive pressure...I'm wondering if the pressure spike is too much before the bullet leaves the case or shortly there after is causing the ejector marks? If I go to a faster burning powder , I'm thinking I will have the same issue.

Kind regards,
Jared

 

[sawgunner2001]

Re: Question regarding powder burning rates

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: JBomb</div><div class="ubbcode-body">Thanks for the responses guys!

I guess I'm more curious to the factors that cause excessive pressure...I'm wondering if the pressure spike is too much before the bullet leaves the case or shortly there after is causing the ejector marks? If I go to a faster burning powder , I'm thinking I will have the same issue.

Kind regards,
Jared</div></div>

You will likely still have the ejector marks regardless of which powder you use. The cause of the ejector marks is the case still being under pressure when the bolt rotates to come unlocked from the barrel extension. This is a non-issue with a bolt gun since before you rotate the bolt manually to extract the fired cartridge, the bullet is LONG gone and pressure has deflated to ambient air pressure. As far as I can tell, the only way to rectify the problem is with a smaller gas port/adjustable gas block...or resign yourself to the fact that ejector marks are inevitable and take your time to find a safe load monitoring the only three reliable pressure signs in an AR, primer flattening, loose primer pockets, and lastly using common sense and not loading WAY over the top.

 

[1lnbrdg]

Re: Question regarding powder burning rates

Some of the bolts have a "slight" raised area around the ejector boring which causes the marks no matter what. The only way to resolve that is have it chucked in a lathe and trued up. No way for me to tell from here, but take a paperclip and run the end around the hole where the ezector is (without the ejector in the bolt) and see if you can feel a high-spot. I could on mine.

 

[command450]

Re: Question regarding powder burning rates

my lyman 49(a must buy for the reloader,IMO)has 114 powders listed in appendix D, for burn rates #1 is the fastest,#114 the slowest.

Also,when you look at once fired brass out of 308 ar's it does take a beating on the cases. I hear mixed opinions on varget.

IMR 4064 is good choice(but the burn rate is one step below varget#80 per lyman 49 manual). alota guys like H4895 which is faster at # 72. H-4198 is at #60 which would be quite a bit faster

never heard anything bad about H 4895

 

[targaflorio]

Re: Question regarding powder burning rates

Jared.: you left out an important piece of info, what bullet are you loading? If it is 175 grs or less, you have a wierd situation. Something else is wrong besides the powder. Are you measuring headspace?
Ejector marks are usually caused by the brass flowing into the open space and then being cut off when the bolt rotates. That is very high pressure and you should back off and reevaluate.
If you post your procedures and measurements we might be able to help.