Have been looking at some match ammo for AR, seems 77gr is popular, but I have heavier buffers opposed to the standard CAR buff to where anything below quality 55gr 223 will "likely" risk shortstroking but 55gr and 62gr 556 work great.

Wondering if a heavier round like 77gr (for example) provides MORE gas pressure to the buffer system, or LESS than standard 55 and 62gr- 556?

Don't have any 77 and just an idea, but if I were ever to find some for a deal, I'd like to know in advance if  could potentially have 223-like pressure after the round is fired, due to the heavier projectile???

Just don't want to ever risk buying something that isn't compatible with a current setup, ya know.

(The IWI Razorcore stuff sounds great, but have seen many online report of pierced Primers happening multiple time per mag, especially in newer lots so haven't decided if want to go that route or not)

1/7" twist btw.

I guess I should ask,

If 77gr just a hot as 62gr?

(In terms of rearward pressure being with a heavier projectile, I know kinetic energy plays a role but didn't know if that dampens the internal blowback.

As long as it still had the cycling pressure of 556 (whether 55 or 62) it should be fine!

Any objections before I consider this a closed topic?

Forsure,

Was just wanting to make sure that the heavy projectile of the 77gr doesn't dampen the gas blowback pressure compared to 55gr and 62gr "556" being likely the same powder load, unless hotter.

Just wanted to make sure the heavier round doesn't dampen the gas given to the buffer system with 77gr type if I ever press go on some.

Speaking mathematically, 77gr would have more back pressure vs 62gr and 55gr, if everything else was equal.
77gr is a longer bullet, so it will have more surface area squeezing against the bore, therefor more friction causing more pressure. The small difference in pressure is probably irrelevant though?

I think you’re over thinking it. You should be good.

Way over thinking it.  If your AR won't run 77 grain ammo then something is wrong with it or you have done something to it that makes it borderline reliable.

Topic Moved

If your rifle reliably cycles 556 ammo, it'll likely cycle any 556 pressure ammo. No need to over think it.
Also the recent lots of IMI are much slower, the m193, m855, and 77gr razor core have all been downloaded, so no more popped primers

Don’t know what the chamber pressure is but IMI 77gr felt hotter in my subjective opinion compared to regular 55/62gr.

A side note, 75gr Gold Dot .223 is the only ammo so far to eject at 1:00 where as M193 is 3:00 in my 14.5” BCM midlength that is a well gassed and soft shooting gun.

@bfoosh06

Forsure overthinking, just wanted to check before "possibly" investing.

Currently, the weakest I own is PMC 223 55, and the setups are an H2 (or a Spikes T-2 which is like an H 1.5 being 4.1 oz, halfway between a H and H2) in a Carbine with Damage Industries chrome silicone spring, and an H buffer in a Middie with factory spring (may switch to Damage Industries on the Middie but want to field test first being the chrome silicone springs feels half the power when hand cycling, although the factory swear they are factory strength, and say it's due to the larger diameter of it and that the factory springs feels more powerful going forward due to "snaking up" when pulling back the charging handle) and day when firing, it won't have the same affect.

The Damage Industries spring also allows the buffer to be fully vertical, opposed to leaning over at the top with many factory springs, likely due to the ends of the springs having half a coil making it not seat flat at the end of the tube. I always read great things about Damage Industries springs so threw one in the carbine, just haven't tested. My only concern is that for a spring that prevents bolt bounce due to the larger diameter in the tube, it makes me wonder if using a Spikes T-2 (non weighed buffer that creates more bolt bounce) inferior to the system and if to just use an H2, if it will work with PMC bronze.

I always forget to bring the PMC 223 to test, but most people who own the same carbine (Mp15) say it's overgassed but assuming the Spikes T-2 or H2 should be fine, but need to do a "non shouldered" last round bolt lockback test with it.

Also have Xtac is both common flavors. I try to not go higher than an "H" for Middie being they are normally not as Gassy due to the longer gas system. I've just gotten into tuning shit before I even try with the CAR buffers being my smith says he runs H2s in "Everything", although not sure what ammo he uses, which is a variable...I know.

I do NOT own any IWI Razorcore, was just something that I saw for sale that seemed exclusive, but read about popped primers being an issue, which I've read has been resolved.

I know PMC also makes 77, and also Federal (which I'm still unsure of what technically is semi-wrong with them being "mil reject ammo" when Federal/AE/Lake City, etc... Yet many trust their life to them.

One reason I TRY to stick with PMC, it's not US made, but I hear it's super reliable and it's also not "mil reject" which concerns me a tad if it were in a mag for SD.

Saw some Federal 77 otm at a dollar per, and being it's Mil-Spec, almost jumped, but first curious as to WHY it's rejected ammo.

(Side note, I noticed a couple rounds of PMC 62gr I have to be a tad different in where the vertical marks on the projectile meets the neck of the casing.

Most have half of the hash marks showing at the neck, yet one round in a box has it fully covered, and another round showed twice as much of that area (showing me how much of the projectile has those hash marks, so gauging from that)

Not sure if one is pressed deeper and the other isn't pressed in enough, although the overall length "appeared" the same. Yet the majority just show a little bit of the hash marks.

That make me wonder if one could produce a squib, and if the other would likely be a flyer.

Do you mean the cannelure on the bullet? There is going to be slightly variances between bullets, especially OTM or hollow point bullets because the noses are not very consistent, so load length will vary slightly, it's normal.
Also any of the military reject Federal xm ammo is good to go. It's all very reliable and high quality. The reasons for getting rejected are very small and unimportant for consumer use. Federal ammo is top tier, and they will not knowingly put out a bad product.
Also, PMC bronze ammo isn't particularly great. It's under powered and rather dirty, and ive actually seen it blow up two guns. The Federal ammo is MUCH better quality than pmc bronze

Wow, that is some seriously next level  overthinking. The only way to really know would be to get a 20 round box of the IMI 77gr and see how it works in your gun.

Also, afaik there is no Federal milspec 77gr. I'm pretty sure that Black Hills still has the contract for Mk262 Mod 1

@MfK

May be the name, it was on "Xtac 62gr" where the hash marks on the bullets that normally show a little (mm or so) at the neck area, in the "same 20rd box", one was where u could see those little lined lines fully, as if not pressed far enough, although the length appeared the same in comparison (like 3x the amount you see of those marks opposed to the rest of the box,
Then one round u could not see the marks at all.

Just being the tedious type like u can see that I am, made me wonder if one was pressed in too far and of one was pressed in too shallow, or if those lines can be on different places of the projectile¿ (Same lot, same 20rd box actually)

Also, I try to stick with X-Tac if PMC for the extra kick, was just saying that PMC 223 is what I consider the bottom of the barrel (although many claim they get amazing groups with it)

I have a little left (not excited about it or anything) from before Xtac came out, or maybe it was before I realized their was an Xtac 55gr, ten or so years ago, not sure which it was, whatever the gunshow had before I was into forums and the online scene to see actual variety and not just what a vendor's table had as a novice (which in this area, I still am), expert in some areas, that's why I'm so OCD being I like to learn what I don't know.

I don't use steel case as that's what I consider the bottom of the barrel, atleast for AR, unless for AK, then it's "standard". I'd think (or LIKE TO THINK) that PMC bronze 223 is more consistent than steel cased 223, as far as doing a lock back test on maybe 5 rounds giving me an answer, opposed to russian stuff where one can blow a flame from the muzzle and the other not even throw a spark.

Although I keep hearing "Wolf Gold" is pretty good bulk ammo, although maybe it's brass and not steel, hence the "gold" in it's title, have never thought to look into it.  I normally lump the "ruskie" made stuff into the same category of "inconsistency" (for AR atleast, for AK, it eats it up and is just the nature of owning an AK) unless you choose to buy better ammo they same as for an AR (although with today's pricing, not as easy.

^

That hashed band is a cannelure. Theres some variation in depth. Being too deep will increase pressure, but yours sounds fine. PMC xtac is good quality ammo, it is the bronze that is the cheaper option. Yes, pic bronze is better than steel case.
Also, wolf gold is very good ammo. Its brass cased and made in Taiwan, by the same factory that produces their military ammo. It is clean and surprisingly accurate, and very cheap as well.

OP:  
You are way way over thinking this............  
Factory 77's  in a reasonably set up rifle will work fine.
I'm not reading your set up paragraphs... it's too long..........  
So I'm just going to assume you didn't over think your rifle and some how bugger it.

Factory 77's have always shot well for me......  
Buy a box.... test it and be done.

OP....your OCD is excessive. Stop chasing all of the "what ifs" regarding every aspect and do some shooting.

As others said, quit thinking so much. Buy some and start testing.

FWIW the POI on some 55gr ammo is very close to 77gr, especially closer in. In a rifle zeroed with 77gr projectiles I'll also use 55gr loads for varmints inside of 100yds as the cost per round is .14 vs about .50 and the POI is very close. However, to find out, you'll need to test. Here is an article about it, written by a fellow Arfcommer: Lothaen`s blog In short, full power loads near m193 specs will likely be very close. The loads I referenced above are handloads with 77gr TMKs and 5tgr Hornady SPBT.

Very misleading post.  If you read the article linked in the post above, you'll see that the author even admits that he completely re-zeros his rifle when using the different loads he mentions.  Not only does he re-zero his rifle when using the different loads, he re-zeros the POA = POI for different distances for the different loads.


From the linked article:


"the M193 was zero’ed at 100 yards and my 75 grain pill was zero’ed at 200 yards."


Even after completely re-zeroing his rifle for POA = POI for two different distances for the different loads, the trajectories of the M193 and Hornady 75 grain differ by 1.8" at 125 yards.







...

Another "You are overthinking it" vote.

Apologies, there was no intent to deceive and I have read the article multiple times. As stated in my post, to find out you'll need to test. In testing you may find that some loads are very close, which is exactly what I found by testing my own loads (much closer than 1.8"). Use different zeros for different loads of course but the intent was to show that it may not vary that wildly and is even close enough to be useful outside of competitive disciplines

That said, I perhaps should have refrained from sharing that altogether as it is tangential to the OP`s post re: pressures.

Glad to see you back as of late, Molon.

No apologies necessary.  

I just didn't want newbies to think that they could zero their rifle with M193 and then expect 75-77 grain loads to shoot to the same POI.


..

What if the M193 was also zero'd at 200?

You can plug it into a ballistics calculator to get an idea, but you actually need to go out and test it to know. You may get lucky and have only vertical changes in dope or it may require horizontal and vertical adjustments to re-zero. You also really need a chrono to find out what that particular ammo does out of your rifle.

Per your question, details are needed to make an educated guess. Testing will tell you the real story.  Every barrel is a story unto itself, although some are more consistent than others. Test.

Many people seem to think that the difference in the points of impact between two different loads fired from a rifle with the same windage and elevation settings on the sights are due solely to the difference in the exterior ballistics (trajectory) of the two loads. This notion is false.  There isn’t a ballistic program on the planet that can predict these changes in the points of impact and trying to do so using the different muzzle velocities and different ballistic coefficients of the two different loads is a wild-goose chase.  

The difference in the points of impact between two different loads (with the same sight settings on the rifle) out to say 75-100 yards will be due in large part to matters of interior ballistics; barrel harmonics and recoil vectors for instance. These components may not be the same from rifle to rifle and are not predictable using ballistic software. The only way to know what the difference in the points of impact will be, is to shoot the different loads in your weapon.

To illustrate the above points with a specific example, the graph below shows that there is nearly no discernable difference in the trajectories of the Sierra 55 grain BlitzKing and the Hornady 70 grain GMX out to 100 yards.





Yet, when I actually fired the Hornady 70 grain GMX load from an AR-15 that had been zeroed for POA=POI at 100 yards with the 55 grain BlitzKings, the group of the 70 grain GMX impacted 2.9” LOW and 3.7” TO THE LEFT of the point of aim.





...

I appreciate all of the advice everyone!

Agreed, Overthinking forsure as I often do, but ammo being so pricey is a reason I asked before entertaining 77gr when 193/855 works fine, so haven't even thought of testing anything lately if bought.

Just didn't want to buy 77gr for SD (being OTM) and it fail due to my "beyond CAR" buffer setup/setups. It sold out so I'll stick to what I know works.

Being my buffer weight setup is to where I go to the highest buffer weight that will shoot pmc 223 "non shouldered" and lock back the bolt catch, then step it down a buffer weight and call it good, so I just had to ask!

Using Damage Industries springs vs factory in these tests has made me question some things but that's a whole nother can of worms.

(In short, The larger diameter and material of those springs makes it feel like it doesn't have much forward strength, but many say its due to the larger diameter of the spring not "snaking up in the tube" when hand cycling a Chrome silicone VS a SS factory spring, making factory springs feel twice as heavy going forward.. by hand atleast)

I now have my answer though on this matter! (Seriously)

I write for a living and always striving for a cookie cutter answer to something where many options/variables are at play in this hobby, which I'm learning to live with. This ammo crisis sucks ass as I understand even the same platform type from the same Lot can perform differently. Especially ammo, so understandably no Direct answers that can be exact!

Thanks again all! Over and out

If I tune a rifle to shoot 5.56 M193 and 5.56 77 grain Black Hills, with a JP LMOS low mass carrier and adjustable gas block... and then I shove in .223 ammo and a full mass carrier... the only thing that happens is 100% cycling and a 100% failure to lock the bolt back on the last round. Function and accuracy do not decrease. That is a way bigger mass shift than a carbine vs H or H2 buffer.

Just a note... my hottest factory ammo is 77 grain. My weakest factory ammo is also 77 grain ( Wolf steel cased .223 77 grain). Ammo weight of 77 vs 55/62 is no concern for function in 5.56 pressure.

I’ve been using IMI and CBC 77gr for years.  

It’s decently hot ammo. It runs just fine in any AR that runs 5.56.  There should be no tuning needed.

I’ve messed with a lot of springs and buffers and I can not so humbly say that if it’s not a reputable mil-spec carbine spring with an H2 or H3, or an VLTOR A5 with an A5-2 or A5-3 buffer, it’s likely some dumbshit spring and buffer that is a reliability downgrade.  Sprinco springs included. Spikes powdered buffers doubly so.