Managing Component VariationsBy Grandpa Mac
- 24th Mar, 2020 Mar 24, 2020, 9:03 AM
- 0 Comments
I read with interest Matthew Cameron’s article on safe loads. Consulting current load data sources is always a good idea and lets us spot any that are out of line. Recently I have been working up some loads for my 6.5x55. This cartridge must be the poster-child for variable load data and the more you look the more confused you become.
A different approach is needed. Several articles by John Barsness on pressure and load development were published in Handloader Magazine over the last decade or so. These boil down to two basic principles:
- Velocity is directly proportional to pressure, all else being equal.
- Velocities increase at one fourth of case capacity, once again all else being equal (see below).
This article is about how we can use these principles to navigate our way through often conflicting data.
Two sets of data were selected for the 120 grain projectile in the 6.5x55mm; Hodgdon’s (as I have used it before) and Nosler as it showed some pretty spiffy velocities.
Both sets of data were developed for the Nosler 120 grain Ballistic Tip projectile so one variable was eliminated. The following data is available on the internet.
|Hodgdon Data 6.5x55mm 120gr Ballistic Tip|
|Start Load||41.5 grains||AR2209/H4350||2545||36,800 C.U.P.|
|Max Load||46.0 grains||AR2209/H4350||2792||45,800 C.U.P.|
|Nosler Data 6.5x55mm 120gr Ballistic Tip|
|Start Load||43.0 grains||AR2209/H4350||2740|
|Mid Load||45.0 grains||AR2209/H4350||2870|
|Max Load||47.0 grains||AR2209/H4350||3000|
Splitting the difference between the 45 and 47 grain Nosler loads (ie; 46 grains) gives us 2,935 fps, about 140 fps faster than the same load in the Hodgdon data. They both can’t be right. Can they?
Previous development of loads in the 6.5x55mm for the 129 grain Hornady SST had shown a variation in burning speed with AR2209 but not this much, so I loaded a short test series with the following results:
|Test Loads 120 grain Sierra spitzer flat-base|
|Test 1||46.0 grains||AR2209 Original||2,806 fps||T3|
|Test 2||46.0 grains||AR2209 Old||2,798 fps||T3|
|Test 3||46.0 grains||AR2209 Current||2,870 fps||T3|
|1993 load||46 grains||AR 2209 Original||2,781 fps||M38|
The “original” and “old” lots are close together and match the Hodgdon data rather well. These lots were manufactured in 1991 (original) and 1998 (old) around the time Hodgdon's data for the 6.5x55mm was developed. The new AR2209 speed falls 65 fps short of the 2,935 fps predicted from the Nosler data, perhaps due to the shorter Tikka T3 barrel and longer throat.
VARIABLES IN LOAD DATA
Each set of load data is like a snapshot in history. Loads were developed using the components available and to the pressure standards in use at the time. All of these vary over time including the pressure standards. In preparation for this article I chose data from old and current sources for the 6.5x55mm plus some on the high and low side of powder capacity for this round.
Only Hodgdon lists pressures and the actual projectile tested. It appears that the Nosler data has been developed with the faster AR2209 and is mostly developed to a higher pressure. One of the significant variables in the data is projectile type. Thin skinned projectiles like the Nosler Ballistic Tip, Hornady Interlock and others tend to produce higher velocities and lower pressures than bonded projectiles like the Nosler AccuBond and others.
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Projectiles with no lead core such as the excellent Barnes produce higher pressure and lower velocities, again with the same load. Hodgdon sells powder but not projectiles so their data includes a mix of projectile types. The Hodgdon data shows these differences. It is important, for this reason, to use data developed for the actual projectile rather than the generic data found in the older manuals.
WHAT VELOCITIES SHOULD BE POSSIBLE?
The .260 Remington is often considered to be a modern equivalent of the 6.5x55mm. The 6.5x55mm’s net case capacity however is around 12% more than the .260 for the 120 grain projectile, thus based on John’s principle of possible velocities increasing at one fourth of case capacity the 6.5x55 should be capable of around 90 fps more velocity than the .260 at the same pressure - or the same velocity at a little less pressure.
The top Nosler load of 47 grains of AR2209/H4350 for 3,000 fps seems to be about max. In the T3’s 40mm shorter barrel this would work out to 2,940 fps.For more recent cartridges that are loaded close to industry maximum pressures things are not as complex.
“Hodgdon sells powder but not projectiles so their data includes a mix of projectile types. … It is important... to use data developed for the actual projectile rather than the generic data found in the older manuals.”
Simply collect the most recent loads for your cartridge, projectile and powder. Make sure it is the same powder, not an equivalent from one of the charts. For our example ADI/AR2209 and Hodgdon H4350 are virtually the same. Other powders including IMR4350, and definitely ball powders, are not.
If you average the results for velocity and charge and adjust for barrel length you will have an indication of your probable maximum velocity.
Back when I worked for a living I used to use graphs and charts to provide a visual representation of data. These days I use them for my handloading data.
Velocity and by association pressure are plotted on the vertical axis with powder charges plotted on the horizontal. Pressures and velocities increase in direct proportion to the charge for single base powders at least. Likely velocity for intermediate loads can be read of the plotted lines.
VERIFYING THE DATA OF YOUR COMPONENTS
If you are going to use a graph, plot the start and max loads plus intermediate loads (if listed) and draw a line through them. Make up a test load, 2 or 3 rounds, using the start load and chronograph them.
I had done earlier development with the 120 grain Nosler projectile and the H4350/AR2209 powders listed and felt confident enough to use a higher load. Do not do this if you are starting out with the cartridge.
Next plot your velocity on your graph and draw a broken line through it parallel to your factory data (see my graph). Much will become clear at this point. Next you can work up toward the factory max. Chronograph as you go and stop if you get a sticky bolt lift or obvious rub marks on the case head.
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Do not exceed either the max load or the max velocity (adjusted for your barrel length). Sticky bolt lift or case head rub marks can be due to a lot of other things rather than high pressure but should not be ignored regardless.
Those who've read this far (congratulations) may have noted that primer appearance has not been mentioned. Some believe that you can divine pressure by looking at the firing pin impression and the edge of the primer. I am not one of them. Sure, I did it myself 40 years ago, but no longer.
There are just too many variables. Certainly you can tell that one load creates more pressure than the last but you already knew that, you added extra powder. You can’t tell how much pressure. To me a much better result can be achieved using a chronograph (remember pressure and velocity are linked) but also use the best, most recent load data available.
To me a chronograph is an essential tool for any handloader engaged in load development. A less expensive model is fine as is sharing one with a few friends. Mine is an Oehler 35P. I have had it from the early 1990s and have never regretted buying it. Without a chronograph and well researched data you won’t know where you are going and worse, you won’t know when you get there.
A CAUTIONARY TALE
Shortly after my chronograph arrived my friend who I will call George (not his real name) had a rifle in 6.5–06. Data was scarce but some was found in an early Nick Harvey Manual.
An early Mulwex sheet showed 55 grains of AR2213. George worked up towards the quoted max of 54.5 grains of H4831 with the 140 grain Hornady Interlock. George however was using AR2213 which is a little faster so he stopped at 54 grains. Velocity was close to Nick Harvey's claims and accuracy was wonderful.
“Without a chronograph and well researched data you won’t know where you are going and worse, you won’t know when you get there.”
George also worked up loads with 120 grain and 160 grain projectiles with much the same results - the lighter projectiles going through close to the same hole. Life was good. Well it was until George had fire- formed all his cases and started to reload them. Not one case would hold a new primer, all the cases had expanded beyond further use.
The primers looked good, easy bolt lift, no ejector marks, yet the loads were clearly too hot. We shouldn’t be too hard on George, he was using the best data available and worked up carefully to just below that. Pressure tested data for the 6.5–06 only became available in 2003 from Hodgdon and later Nosler. Both listed 49.5 grains or a little more for the 140 grain projectile.
A good thing George didn’t try 55 grains!
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