Jump to content

Short barrel 308, fast bullet, what powder?


meles meles

Recommended Posts

 

Great website I check it every day for their informative and interesting articles.

 

To give an indication of how QL may deceive on this, here are the results of one of the very first tests I did on RS52 using a sample lot provided by Nigel Cole-Hawkins under the manufacturer's OEM description of Nitrochemie EI-N130 in December 2013. There was no loading data available, and QuickLOAD hadn't been updated with it at that time, so the advice was to use Alliant Re15 data as a guide.

 

This had been done from very low levels initially, then a second and even third stage tests were run with higher charge weights once a 'feel' had been obtained for the powder's characteristics. Parallel loads with H. VarGet and Re15 were run alongside in the same sessions using identical components to provide comparisons.

 

The rifle was a Savage PTA based FTR rifle with a 32-inch 1-12 twist Bartlein barrel, minimum SAAMI chamber dimensions and freebore cut to suit long 155s such as the 155gn Scenar seated slightly shallow in the neck.

 

With the short-shank (low barrel friction) Sierra 155gn Palma MatchKing (#2156, later model), loads for all three powders were worked up to 47.4gn in Winchester brass (57gn water capacity = +1gn on Lapua) with Federal 210M match primers. 15 thou' jump. MagnetoSpeed V3 chronograph used.

 

QL prediction for H. VarGet .......... 59,840 psi / 3,024 fps MV .............. Actual MV ............... 2,997 fps

 

......................Alliant Re15 ......... 57,428 psi / 3,042 fps MV ............................................... 2,999 fps

 

..................... RS52 .................. 47,934 psi / 2,925 fps MV (not available then, run today) .... 3,039 fps

 

Great powder though RS52 is and no matter how much benefit the patented EI deterrent infusion process gives, it simply cannot provide another 40 fps over VarGet and Re15 whilst running at 10,000-12,000 psi lower pressures. It almost certainly produced similar peak pressure values to the others, but gave the usual 30-50 fps extra MV I mentioned.

 

Incidentally, the trio produced near identical MV spreads / ES values for 5-round strings, all good with the top 47.4gn charge weight, rather higher with lower charges - 11 fps for RS52, 12 fps spreads for Re15 and VarGet; SD of 5 for RS52 and Re15, 6 for H. VarGet.

 

'Playing' with QuickLOAD to match that 3,039 fps actual MV, the program says RS52 produces that at 49.4gn - ie exactly 2gn 'out' on the wrong side. I regard that as a quite unacceptable margin. The two 'right side' errors for Re15 and VarGet are within typical accuracy levels for what as others in earlier posts point out is a model, and in % variation terms are quite close, VarGet's actual being 0.89% 'out' on the predicted MV, production lot variations often accounting for this sort of deviation.

 

 

VERY IMPORTANT NOTICE. These charges were worked up in a dedicated longer than SAAMI freebore competition rifle with a top-grade match barrel. They may produce excessive pressures in other rifles. DO NOT USE THE CHARGE WEIGHTS SHOWN FOR THESE THREE PROPELLANTS WITHOUT EXTENSIVE WORK-UPS FROM MUCH LOWER CHARGES WHILST MONITORING MVs AND PRESSURE SIGNS SUCH AS HARD BOLT LIFT, EJECTOR MARKS ON CASE-HEAD AND SUCHLIKE.

 

Laurie, how does Nitrochemie's/Reload Swiss' data compare now to your early tests?

 

I did a quick cross reference with their 'pressure tested' and chronographed data published on the 308 in the current (2017) reloading manual and that calculated by QuickLOAD. I used their current (actually late 2014) bomb calorimeter data for QuickLOAD on RS52 and RS60 powders, and the two are very close to each other in both pressure and velocity?

Link to comment
Share on other sites

  • 2 weeks later...

Sounds good. Any idea of velocity?

 

 

Yes, we have just done some velocity testing using this load. The results surprised us: they were quite a bit faster than the Quickload predictions. We didn't expect the calculated velocity to be spot on, but we thought it might be closer than it was. The higher velocity attained reinforces the observation from Laurie that the pressure readings also tend to be underestimated in QL for RS powders though it should be said we saw no high pressure signs on any of the shots fired.

 

Maximum velocity: 2765 feet per second
Minimum velocity: 2727 feet per second
Average velocity: 2744 feet per second
Standard deviation: 14.0 feet per second

 

Link to comment
Share on other sites

Interesting,

 

This backs up what I read before, that a velocity limit was found with a given powder before any pressure signs were seen.

When I say limit, the info said that any further increase in powder weight did not produce any useful increase in velocity.

 

It may be to do with the small contact area on the bullet in the barrel, if it does indeed run on the bands in the rifling and not the main body of the bullet.

 

I really need to find that info I keep babbling about. I'll scour the net when time permits and see if I can find it.

 

Excellent findings so far, very informative. Thanks for posting.

Link to comment
Share on other sites

This is a well known phenomenon, but not one that can be relied on to appear. Anybody who has loaded a large number of component combinations in several cartridges / calibres for several rifles will have found that reaching maximum usable loads is exhibited in one of three ways:

 

1) The charge is increased, but the MV doesn't respond having reached a plateau.

 

2) Pressure spikes rapidly - ie no obvious over-pressure signs on the case-head is replaced by the whole set up to and including blown primers by a normally safe charge weight increment of 1% or less.

 

3) Pressure signs appear sequentially suggesting pressures are increasing in a linear way allied to the charge weight change.

 

As most of us don't have access to pressure measuring kit, I'll stress we're talking pressure indications so we're at a remove from what actual chamber pressures may be doing. I'll also add that primer appearance changes (ie 'flatness') may be, often are, misleading, although a primer leak / blown primer are signs of excessive pressure however one regards this matter.

 

As far as 1) goes, this has to be distinguished from the often 'jerky' way that MV readings increase over a series of charge increase steps. That is if a full grain weight charge increase averages say 60 fps, three or four consecutive 0.3gn increases often don't produce nice neat step changes of ~20 fps, but look something like 2,800 ... 2,812 ..... 2,835 .... 2,862 fps.

 

When the form of maximum charge / pressure / MV plateau relationship appears, not only do velocities usually fail to increase further, one often sees a reduction in MV after a further step of two.

 

I've seen and recorded all three forms many times over the years, but form #1 is the least common by far and I cannot suggest any guidelines as to what sort of combinations might produce it other than it appears to be much more likely when the powder being tried is on the slow burning side for the application and is rarely if ever seen if a bit too fast burning grade is being used.

Link to comment
Share on other sites

Laurie,

 

You may find some of your weird internal ballistic effects observations, after the type of propellant, projectile, case and primer used, are due to these three main factors:

 

1) Charge position within the case;

2) Ullage, i.e., the empty space in case not occupied by the propellant charge or the bullet;

3) Propellant morphology (shape).

 

The first can have dramatic effects, once the propellant is ignited, it can cause irregular burning and severe pressure waves to form. This most often erroneously described as a propellant detonation, but is most likely in fact due to pressure waves travelling backwards and forwards between the base of the projectile and breech face (cartridge web). The waves can reinforce, causing very high pressures to form, the pressures so high that the breech fails or projectile fails (well in big guns). Detonation of the propellant will only occur when shocked or pressurised in the 10's of kilobar (>145 tons/inch2 - TSI) region. These pressures are not reached even when loading full cases with the fastest pistol propellants. Detonation of the propellant charge within a case and barrel would cause different deformation (failure of the materials) effects than seen on firearms that have reportedly failed by propellant detonation. Due to the velocity of the detonation wave and density of the explosive material, detonation causes pressures in excess of 300 kilobar (approximately 2,175 TSI), so way above the strength of most materials. The pressure of such a detonation wave can be roughly calculated using dynamic pressure equation, so ½ x density of the liquid x square of its flow velocity.

 

The ullage can cause the above pressure wave problems if it's large. At moderate ullage levels a flash channel can form over the top surface of the propellant as it settles under gravity. This can cause varying ignition characteristics and burning rates (pressure generation), thus velocities depending on the amount of surface area present at ignition. Lower ullage levels can have result in ignition problems depending on if the ullage space is at the rear (case facing down before loading), or at the front (case stored in the upward direction before loading). The former results in the flash from the primer having to travel across a gap to the base of the propellant charge, expending energy which would be better pushing the flame and incandescent particles into the propellant charge matrix to insure reliable ignition; this can be one cause of hang fires. The latter may cause a differential pressure, or force on the back areas of the projectile and this may result in a slight initial yaw as the projectile is forced out of the case and into the barrel region. This effect is more apparent when the base of the projectile intrudes some distance past the neck/shoulder interface of a case.

 

The shape of the propellant varies the way the propellant both ignites and flows. The grain shape varies the way the gases from both the igniter (primer) and the burning propellants propagates through the propellant matrix, thus changing its ignition characteristics. The shape also affects how the propellant grains flow when pressurised. Most propellant grains flow from the case into the bore after being ignited, the amount flowing in dependant on their burning rate, shape, the case shape, along with a whole host of other factors. The general rule is that the more irregular the shape the more viscous the flow. This effect can be seen simply by pouring propellant into a funnel with a set tube output diameter and watching. Some can be seem to flow cleanly under gravity (ball propellants), so flow more slowly (small sticks/tubes), some don't flow very well at all (flake and especially Trail Boss) and many may result in choked flow when the flow reaches a critical value. The way the propellant flows from the case will vary with load density and pressure, hence this may give weird effect high-low velocity figures at critical values.

 

These effects are seen in large weapons such a cannon (20-57 mm), artillery and tank guns (my background) and small arms too.

 

There are plenty of books, presentations and papers on the subject for those who fancy reading about it.

 

BB69

Link to comment
Share on other sites

Thank you for that full reply. However, most of it has no bearing on the findings I mention. I don't think I've loaded a rifle cartridge with standard loads in over a third of a century that had a lower fill-ration than 80%, and precious few will have been under 90 or even 95%. So charge position effects just didn't apply.

 

Likewise, the difference between ball and extruded types aside, smallarms powders have relatively insignificant differences in size / shape these days. Manufacturers have chased the Holy Grail of short-cut powders so successfully in recent years (for more consistent powder metering by both commercial and hand loading users) that size / shape variations have reduced greatly, and performance / application is no longer determined by grain surface areas. For instance, Hodgdon quotes the same loads for original (long-grain) ADI manufactured H4831 and its later H4831sc small kernel shortcut version. In a quarter century of using the pair, I've found few if any performance / behaviour issues in a variety of cartridges other than how much space they take up in the case.

Link to comment
Share on other sites

Interesting,

 

This backs up what I read before, that a velocity limit was found with a given powder before any pressure signs were seen.

When I say limit, the info said that any further increase in powder weight did not produce any useful increase in velocity.

 

It may be to do with the small contact area on the bullet in the barrel, if it does indeed run on the bands in the rifling and not the main body of the bullet.

 

I really need to find that info I keep babbling about. I'll scour the net when time permits and see if I can find it.

 

Excellent findings so far, very informative. Thanks for posting.

 

The bullet does have driving bands, and being brass rather than lead we would image theyt don't deform and swage down significantly and so the bullet does probably ride on them. We'll take a bullet to the lab at work and do some hardness indents on it.

 

Regarding the velocity / SD we obtained with our load, we feel it's quite adequate for our needs: 0.5 MOA is pretty good for us. That not withstanding, what do better shooters expect to see in terms of an average velocity & SD from a .308 ?

Link to comment
Share on other sites

Thank you for that full reply. However, most of it has no bearing on the findings I mention. I don't think I've loaded a rifle cartridge with standard loads in over a third of a century that had a lower fill-ration than 80%, and precious few will have been under 90 or even 95%. So charge position effects just didn't apply.

 

Likewise, the difference between ball and extruded types aside, smallarms powders have relatively insignificant differences in size / shape these days. Manufacturers have chased the Holy Grail of short-cut powders so successfully in recent years (for more consistent powder metering by both commercial and hand loading users) that size / shape variations have reduced greatly, and performance / application is no longer determined by grain surface areas. For instance, Hodgdon quotes the same loads for original (long-grain) ADI manufactured H4831 and its later H4831sc small kernel shortcut version. In a quarter century of using the pair, I've found few if any performance / behaviour issues in a variety of cartridges other than how much space they take up in the case.

 

Charge position most certainly does affect propellant combustion at all levels, as does grain type and morphology. Whether you or others if the target shooting arena have observed anything on the crude scale of just a single observed velocity and groupings at a certain distance is another matter.

 

The info in the post was taken from very recent, along with older papers, presentations on internal ballistics. This along with observations I witnessed when testing weapon systems undertaken with complex instrumentation (multichannel and multiple types of velocity measuring system, multichannel pressure measuring systems, multichannel strain measuring systems, multichannel flash x-ray and high-speed [video, digital and cine] photography).

 

Some of the recent observations of ullage and its affects on the internal ballistic cycle are in fact on smallarms themselves This research was undertaken by the US military itself, most commonly the US Army Research Laboratory (ARL), though contractors had done so too.

 

If you want to dismiss these facts and my observations, so be it, but these are facts.

Link to comment
Share on other sites

 

The bullet does have driving bands, and being brass rather than lead we would image theyt don't deform and swage down significantly and so the bullet does probably ride on them. We'll take a bullet to the lab at work and do some hardness indents on it.

 

Regarding the velocity / SD we obtained with our load, we feel it's quite adequate for our needs: 0.5 MOA is pretty good for us. That not withstanding, what do better shooters expect to see in terms of an average velocity & SD from a .308 ?

That would be interesting.

 

If the results are ok for you then it's all good. I'm not a top level comp shooter and I would certainly be happy with your results. I would imagine the velocity would take you out to 800 yards easily?

 

On another note, would these bullets have any noticeable affect on barrel life and are there any restrictions shooting these on any particular ranges, MOD for example?

Link to comment
Share on other sites

The MoD don't allow solid bullets on their ranges. I use these on a private range.

 

I would expect barrel life to be quite good, possibly better than with conventional copper jacketed bullets. The bearing surfaces are small and the brass is free machining and thus probably quite lubricative.

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use and Privacy Policy