OzFclass

G'day All. This question's one for the experienced moly users I think.

If "batch A" bullets were being wet coated for 2 hours and had x amount of powder added, and then "batch B" were coated for 4 hours in the same solution, months later, with a little more powder and water added, will:
a. The thickness of moly keep increasing the longer its being tumbled?
b. Will this cause a difference in MV between batches given all else is equal?


The story behind this is. In January I developed a brilliant load for my new FSTD rifle (10T). Moly coated 155 HYB, 46.6gn AR2208, FedGM210, Lapua large primer cases at 3000 fps. The node was from about 2980 fps through to 3020 fps.
As the temps been cooling off over the year and the round count has been increasing to 600 rounds, I've been watching the MV deteriorate. 46.6gn is now at 2960fps.
I have been putting it down to the seasonal change. I anticipated that when I got around to tuning a winter load, I might be up 0.2 - 0.4 gn from where I was.
I went to the range yesterday to find a winter load and shoot some 5 shot groups from 46.6gn through to 47.4gn in 0.1gn increments.

Using a labradar, 47.4gn is only hitting 2990 fps now. The node is forming at 47.1gn but still getting tighter at 47.4gn. Primers are starting to show pressure signs so I wont go any further.

From the January's load development to yesterday, I have been using the same brass, primer batch, powder batch. The only 2 differences are a new batch of bullets which measure only .002" longer base to ogive and the fact that I've moly coated them, in the same solution with a tea spoon of powder to top up the mix. Also had them in the tumbler for 4 hours rather than the original 2.

I believe that some of the MV deterioration is seasonal, but unsure of whether a possible difference in moly coating will reduce speed further or the extra MV deterioration is more likely just the barrel bedding in.

Hoping someone might have some ideas or experiences to share.

Thanks.

I always shoot my mollied projectile within the batch they are done, I have never seen a performance difference from batch to batch.

Up at those speeds Denis I am wondering why you are not running palma brass?

I think .308 barrels take a lot lot longer than people realize to settle in and respectively go off again, where a 284 settles in about 180-200 rounds, maybe a few more, I think you can easily double that and more, I've had a few barrels that just seem to keep getting better and better all the way up to around 1000 rounds, and last well beyond 2500-3000 rounds of good competition life, even with heavy pills.

I have 3 barrels I am currently running in and developing which is helping affirm this belief.

I have seen more than a couple of people make the mistake of barrel break in, load develop then shoot without monitoring and wonder why things go pear shaped, I think with 308's you need to be monitoring closely right up to and possibly beyond 300rounds, my last barrel had a sudden jump of 70fps, if you are not ontop of things this will easily confuse you, this particular barrel up until about 120 rounds was a hummer, shooting sub .5MOA at 1000yds almost out of the box so to speak, it has taken quite some time to get it to settle down again.

id ask a couple of questions to also help work out what is going on.

1) how compressed is the load? i.e. what is your seating depth/freebore

2) what length and what brand of barrel.

3) you seem to be at the point of diminishing returns for more powder, if I read it right close to a full grain is giving you only about 30fps, I am guessing the load is compressed? Have you considered trying another powder?

4) are you storing your powder correctly? (I am sure you are, but worth asking)

Looking forward to others comments on this one.

Hi Jase,

I agree that I must be starting to compress at the 47's. At 46.9gn I could still hear a few of kernels shaking around. I don't want to be up over 47gn. I'm just trying to understand where the difference in MV is coming from.
I bought bulk powder in 500g tubs so it's definitely being stored correctly in terms of not being subject to the atmosphere and moisture.
I may have to go to ar2206h to get the node I want, but I prefer slower powders where they can be used.
The reason I've gone for moly and ar2208 is so that I can extend brass life of LR primer cases. It's working in that I have 8 firings on 2 batches (that were fire formed with light jammed loads to work harden the base) and the PP are still VERY firm.

This is a 30" barrel chambered with a Bisley reamer. Not sure of the specs there. But at a .030" jump there's probably only 3 - 4mm of shank in the neck and the base of the bullet looks to finish at similar length to the shoulder/body junction.

Excuse my ignorance on the subject but is it normal for Loads to be higher when using Moly? I only ask as those loads sound pretty hot compared to 46.1 of 2208 that i use with un coated bullets.

Jase PTRC wrote:Excuse my ignorance on the subject but is it normal for Loads to be higher when using Moly? I only ask as those loads sound pretty hot compared to 46.1 of 2208 that i use with un coated bullets.

Yes, in general it is, but your load will depend on a vast majority of factors, bullet shape, barrel bore dimensions, chamber spec etc. etc. I ran 46.8gr naked in one of my 155gr barrels, it shot amazing for 1000's of rounds and was eventually given away to a friend who was starting out. I still have the cases from that rifle with plenty of life left in them.

Jase PTRC wrote:Excuse my ignorance on the subject but is it normal for Loads to be higher when using Moly? I only ask as those loads sound pretty hot compared to 46.1 of 2208 that i use with un coated bullets.

Generally moly coating will reduce bullet friction and therefore chamber pressure so the MV is reduced from the same naked bullet load. I believe that most shooters find that when they change to moly coated bullets they have to increase their load by 0.2 - 0.4gn to get the same velocity. Most FTSD naked bullet ar2208 loads I've heard of are close to the 46.2gn mark, so 46.6gn with moly fits the bill.

I measured the bearing surface and gas seal diameters of the first bullet batch and second bullet batch using a .001 dial indicator. Not ideal but it's the finest measuring tool I have here and I can see the needle travel through each .001" increment. The diameters are too close for me to pick apart and I can easily discern .00025" of needle movement so I suspect that there is no difference in moly coating thickness. I'm leaning towards the barrel bedding in causing the MV difference.