Reloading and pressures

[orchemo]

I had my gunsmith spin a 6.5 CM barrel onto a Sako long action and re chamber to a 6.5-284 Norma set to accept OAL up to 3.228 inches.

Most of the loading data has tested OAL at 2.98 inches. I have been seating to 3.15 for AB 140grs and 3.17 for Barnes 127 LRX.

I am finding that my velocity is lower than book which I am thinking is due to the longer OAL verses the book data

I have been using Gordon's Reloading tool (GRT) to simulate the loads using my Nosler brass case volume, barrel length and get very close correlation in velocity.

Question:
1. The powder charges predicted by GRT are all slightly above Nosler data, much above Barnes data (which Nosler data is too)

Example: 125gr Nosler data
Nosler - H4381SC 53.5 gr for 3029 (26" barrel)
GRT - H4381 56.5 gr for 3061 fps (24" barrel"

I am attributing this difference in powder charges to my longer OAL gives the case more volume, thus needing slightly more powder to achieve the same pressure?

Thanks

 

[Idaho53]

A few things to consider: Barnes data will always be different due to their bullet design (solid monos vs. cup/core or bonded bullets), typically longer seating depth will increase pressure/velocity due to less jump to lands, nothing really replaces real live data of a load fired some a specific gun, I have gone over book max. loads before but always watch closely for pressure signs on the brass/primers (assuming headspace is good), your case powder to bullet seating depth can affect pressure and may be the source of GRT's projected load data...although my personal experience is case powder/bullet depth ratio has more of an affect on consistency of velocity between each shot (lower ES/SD)...this assumes the load is not a compressed load (which is another topic)

 

[300WSM]

I had my gunsmith spin a 6.5 CM barrel onto a Sako long action and re chamber to a 6.5-284 Norma set to accept OAL up to 3.228 inches.

Most of the loading data has tested OAL at 2.98 inches. I have been seating to 3.15 for AB 140grs and 3.17 for Barnes 127 LRX.

I am finding that my velocity is lower than book which I am thinking is due to the longer OAL verses the book data

I have been using Gordon's Reloading tool (GRT) to simulate the loads using my Nosler brass case volume, barrel length and get very close correlation in velocity.

Question:
1. The powder charges predicted by GRT are all slightly above Nosler data, much above Barnes data (which Nosler data is too)

Example: 125gr Nosler data
Nosler - H4381SC 53.5 gr for 3029 (26" barrel)
GRT - H4381 56.5 gr for 3061 fps (24" barrel"

I am attributing this difference in powder charges to my longer OAL gives the case more volume, thus needing slightly more powder to achieve the same pressure?

Thanks

Don't ASSUME anything.

Let's look at some data first.

I dont see any velocity listed from your weapon so what is your actual FPS you're finding on an actual chronograph?

Let's start there.

How far below are you getting with your platform and handload versus published data?

Real world numbers not projections from a program.

What's the chronograph actually telling you?

 

[RL338]

I am finding that my velocity is lower than book which I am thinking is due to the longer OAL verses the book data

3.150” vs 2.980” will cause lower velocity due to the extra usable case volume.

 

[Aussie Wanderer]

I actually had the opposite experience in my 300rum where I picked up an extra 70fps by seating the 200accubond further out and improved my accuracy substantially at the same time. This was using Garmin’s little chronograph which has proven to be a good little unit in terms of consistency. I gave it some thought, couldn’t find a logical reason in my head but was happy with the outcome.

 

[jimbires]

there are always an exception to the rules , but this is what I've seen .

all things being equal , more case volume will need more powder to reach the same velocity . seating the bullet longer , increases case volume . now if you are seating long enough to be jamming a bullet into the lands, this will increase pressure . how much it increases pressure , I don't know . I'm sure there are numerous things that come into play . a major one would be bullet bearing length . from what I've seen , as long as I have a jump the pressure doesn't seem to be greatly affected . I'm sure ammo could be loaded ridiculously short and increase pressure, from less volume . I've never played like this on normal ammo , to see if I could tell . I do get into very short seating depths on reduced loads , this is where I often find accuracy with them. being this is a reduced load I don't see any pressure signs from a very short COAL . a simple way of thinking about more case volume needing more powder , is the Akley improved . the AI case has more volume and it will show a higher powder charge being recommended .

 

[TackDriver284]

A few years ago I had the same issue using the H4831sc in my 6.5 x 284 using the 143 ELD-X where my velocity is about 100 fps slower ( I think it was in the neighborhood of 2650 fps ) than book max ( 2768 fps) , due to long OAL length to be closer to the lands. If I could remember, I worked up a ladder up to 49 grains as book max and velocity was slow. I did another ladder up to 52.9 grains, watched for pressure signs and found a good node around 52.3 @2910 fps. It shot real well. The H4831sc is a slow powder but is temp stable.
Factors for low velocity is possibly due to long OAL length, extra case volume and slow burn rate of the H4831sc . If working up another ladder past max book charge in .3 or .4 increments and watch for pressure signs is very important, if you are not familiar in reading pressure, I suggest you learn how to read pressure before running a ladder past maximum book charges to gain more velocity. Another option is to switch powders to IMR 4350, H4350 or RL 22 to get a better velocity that you are seeking. Also different brands of brass has different case volumes, some are less or more than others. Thickness of brass contributes to case volume.

 

[Bruce Mc]

I recently changed brass on my 130 gr Partition load from Winchester to Nosler. The Win case was loaded with 55 grs of Reloder 16 and muzzle velocity was 3110 fps or so. I tried 55.5 gut it badly flattened primers so I backed off. 56.0 was the max load according to Aliants wed site. When I fired off a short ladder test in the Nosler brass I got to 3140 fps with 56.5 grs and still had nicely rounded edges on the primers. I measured the case capacity with water and had the Dr run me a quick load specific to my conditions. It showed the max load still another 2 grs more. In both brass the length dimensions were the same. I also measured the capacity of the Win brass and was a grain less than the Nosler. So my analysis, IMO, is that more capacity, even a small amount, can create less velocity with equal powder charge but when increased actually results in higher velocity. This was in a 270 win.

 

[Blkram]

https://forum.nosler.com/threads/effects-of-temperature-and-elevation.47787/#post-525198

I did a little study recently on the effects of temperature and elevation and posted it here in the thread listed above.
I didn't look at the powders though. Or Pressures.

You are likely aware of the following, but we sometimes forget some of the variable factors...

There may be other variables at play for your rifle...and we must remember that reloading manuals are guides and not absolutes...and there is a factor of safety involved here to protect the manual's authors from litigation...for example, including, but not limited to:

Chamber: the reamer the gunsmith used may be new, or well worn and the variance in your chamber may have an effect, as more, or less, energy is used in expanding the case to your chamber dimension, effecting overall performance.
Barrel bore: Again, the button or cutter used to rifle the barrel twist may have been new or well worn, and have a variance on the pressure results in the bullets going down the barrel effecting overall performance. And how smooth your bore is can affect pressure performance and velocities. There are definitely faster and slower barrels out there!
Case: use of a different case, with a different case capacity, can produce enough variance to impact overall performance from the published data
Bullet: have you tested the actual bullet diameter of the ones you are using to verify that they are actually 0.264"? If there is a variance in them, it will effect pressure. different bullet shapes and materials will impact OAL due to actual variations in bullet length, encroachment in to powder space, jump to rifling, etc,
Powder: The grade of powder used by many ammo companies and bullet makers, isn't always the same grade as what we can by for reloading, and will have an impact on the pressures actually produced in your loads, in your rifle as compared to the test barrel/firearms they use for load development for the reloading manuals. Then there is powder lot variations. And the quality of the powder in use; new or old, how was it stored? Did it take on moisture in storage? Has it degraded? Etc.
OAL: as you have already stated, there is a variance in the recommended OAL and that which you are using that will produce a va
ariance
Throat dimensions: the length of throat will also produce a variance in pressures for your rifle
Pressure: Many of us do not have the equipment to test for actual pressure in our firearms and the various ammunition we are shooting to determine if it safe. Many of us will only have a chrony that measures velocities and gives us an actual for the ammo used in our firearm for comparison to the manual's published data, but it doesn't provide the full picture. And then there is signs of pressure that can be felt or seen (sticky bolt lift, unable to extract the case, flattened/cratered primers, ejector marks on the case head, etc.)...but typically if we are feeling/seeing these, we are most likely well over the safe pressure limit with that load in our rifle

While this may, or may not, answer your specific question...
Hope this might help you in your search for an answer...but good questions to ask if you are talking to one of the technicians at one of the companies who produced the reloading data.
If not, hopefully it helps someone reading this thread with similar questions.

Look forward to hearing what you finally determine in your search for an answer.