What cans are we using on SBRs and AR pistols?

[markm]

Sounds like you've thought about it and this makes sense - but have you considered going just 3" longer? You will have such a more capable weapon for virtually no downsides. I run an AAC Mini4 on a 10.3" Centurion and it is not at all "loud" but it goes subsonic around 550y - considerraabbllyy further than a 7.5". It retains expansion/fragmentation capabilities beyond room distances. Docsherm has shot it - he will hopefully agree.

I'm not trying to chastise you for your choice, but 7.5" 5.56 guns are under the expansion threshhold on almost all types of .224 projectiles.

I agree. Under 10.5 an you're robbing the cartridge of too much velocity.

[WillBrink]

Off the top of my head, the only two cans I can think of that I’d put on a 7.5” is the Sandman S and the Chimera 300.

I’d strongly consider a longer barrel. 7.5” is the Ed Hardy of barrel lengths.

See my comments in #18. When they first came out back in the day, they were crap for reliability and the ammo highly dependent on velocity for its terminal ballistics, so they offered very little of value or interest to me.

That made them like a high cap 1911, to be avoided or range toys at best. Reliability seems to have been figured a long time ago and modern HD/SD ammo achieves acceptable terminal ballistics from very short 5.56 barrels. I'm never an early adopter, or beta tester or toy seeker for guns, cars, audio, phones, etc and research it bigly before making choices. My opinion, which you may not share, 7-12" for HD/SD with appropriate ammo choice at SD/HD distances, will achieve similar terminal ballistics. That was the conclusion I have come to.

Been here 13 years and you're getting a 7.5" 5.56 pistol?

Bruh. Will. My Man...

And that will now make a grand total of two ARs I own!

[WillBrink]

Sounds like you've thought about it and this makes sense - but have you considered going just 3" longer? You will have such a more capable weapon for virtually no downsides. I run an AAC Mini4 on a 10.3" Centurion and it is not at all "loud" but it goes subsonic around 550y - considerraabbllyy further than a 7.5". It retains expansion/fragmentation capabilities beyond room distances. Docsherm has shot it - he will hopefully agree.

I'm not trying to chastise you for your choice, but 7.5" 5.56 guns are under the expansion threshhold on almost all types of .224 projectiles.

At what distances? Tests I have seen using various modern SD/HD intended at typical SD/HD distances resulted 18" + penetrations and expansions, and are above expansion threshold velocities for the bullet. I personally don't give a damn about anything passed say 25 yards, and that's pushing it. Due to modern bullet designs, seen similar terminal ballistics from 7-12 barrels. For example, the excellent Barns TSX has a min expansion threshold 1800fps for the 70gr and
-1900fps for the 55,62 gr.

62gr Fusion apparently 1600ish, unless you have another source on that, but plenty of margin in the 7.5 velocity.

You should easily stay above 2000fps with a 7.5. with a mid weight/62g bullet Not posting as proof, but as reference, but at 2;52 of vid with 7.5" 2400 fps give or take, which should = more than sufficient penetration and expansion/terminal ballistics at SD/HD/PDW distances.

I agree. Under 10.5 an you're robbing the cartridge of too much velocity.

See above. If both result in 18" of penetration and reliable expansion for the SD/HD mission and distances, does it really matter? I care about terminal ballistics, so penetration and expansion within SD/HD distances in a very small package. The rest is mostly immaterial, so shorter the better as long as it produces velocities easily above minimal expansion thresholds for the bullet used at typical SD/HD distances.

[Eurodriver]

This is America - you're free to get whatever gun you damn well choose! But you're talking about a home defense rifle being used to incapacitate an attacker. I'm just sharing data, but it seems you want a 7.5" for no other reason than it looks cool despite your reason for ownership being a self defense role. If you just want 12-18" of penetration and expansion why not use a 9mm with HPs? Less noise. Less recoil. And you'll have a much wider permanent wound cavity.

A huge benefit to a rifle is the temporary wound cavity generated by cavitation with a cartridge traveling over ~2400 FPS.

A whole bunch of data points here:

https://www.rathcoombe.net/sci-tech/...s/methods.html

Cavitation increases considerably (of necessity) as velocity increases (in proportion to kinetic energy), so the penetration is not the only measure of wound lethality.

And some here:
https://www.bevfitchett.us/gunshot-w...listics-1.html

It has been found that above a certain critical velocity 800 to 900 m/sec (2625 to 2953 ft/sec), the character of a wound changes radically with tissue destruction becoming much more severe.2 Trans- or supersonic flow within the tissue causing strong shockwaves has been assumed to be responsible for this effect.

[Defaultmp3]

A huge benefit to a rifle is the temporary wound cavity generated by cavitation with a cartridge traveling over ~2400 FPS.

Well, according to your second link:
It is the author's belief that rather than there being a critical velocity above which the severity of wounds increases dramatically, there is instead a critical level (amount) of kinetic energy loss above which tissue destruction becomes radically more severe. This level is different for each organ or tissue. When a bullet or missile exceeds this kinetic energy threshold, it produces a temporary cavity that the organ or tissue can no longer contain, i.e., one that exceeds the elastic limit of the organ. When the elastic limit is exceeded, the organ "bursts." For full metal-jacketed rifle bullets and steel balls to reach this critical level of kinetic energy loss, these missiles must be traveling at very high velocities (greater than 800 to 900 m/sec; 2625 to 2950 ft/s). For soft-point and hollow-point rifle bullets, however, the same loss of kinetic energy will occur at lower velocities as a result of the deformation and breakup of the bullets. Thus, in the author's experience, for hunting bullets the critical velocity, appears to be between 1500 and 2000 ft/sec (457 to 610 m/sec).

At LAIR, shots were made in the 6000-6500 fps range and the TC was minimal--like a .38 sp LRN for those projectiles which remained point forward.

Shotgun slugs and large caliber deforming rifle projectiles (think old school 1800's era calibers) can have velocities below 1500 fps, yet offer quite impressive TC effects.

[georgeib]

This is America - you're free to get whatever gun you damn well choose! But you're talking about a home defense rifle being used to incapacitate an attacker. I'm just sharing data, but it seems you want a 7.5" for no other reason than it looks cool despite your reason for ownership being a self defense role. If you just want 12-18" of penetration and expansion why not use a 9mm with HPs? Less noise. Less recoil. And you'll have a much wider permanent wound cavity.

A huge benefit to a rifle is the temporary wound cavity generated by cavitation with a cartridge traveling over ~2400 FPS.

A whole bunch of data points here:

https://www.rathcoombe.net/sci-tech/...s/methods.html



And some here:
https://www.bevfitchett.us/gunshot-w...listics-1.html

I was going to say the same thing, but you said it better. Velocity is what makes rifles so much more effective than pistols. If you're giving up all that velocity, may as well go with a PCC, IMO.

[WillBrink]

This is America - you're free to get whatever gun you damn well choose! But you're talking about a home defense rifle being used to incapacitate an attacker.

I view it more as a HD PDW, with the known +/- of that.

I'm just sharing data, but it seems you want a 7.5" for no other reason than it looks cool despite your reason for ownership being a self defense role.

Dude, screw off with that comment. I did as well a job as anyone here, you included, of my reasoning using facts and data. My research suggests such a thing will more than adequate fit my wants/needs as outlined repeatedly.

If you just want 12-18" of penetration and expansion why not use a 9mm with HPs? Less noise. Less recoil. And you'll have a much wider permanent wound cavity.

That is a legit counter point, but multiple points of contact, higher capacity, ease of use, unless you mean 9mm carbine? I did address that in another response. I did not do any head to head research on a AR carbine in pistol calibers and compare numbers. I may go to a 300blk upper at some point, and thought I'd start with 5.56. Data I have seen is depending on load, you're hovering right around that perm vs temp would cavity velocities. While I don't claim expertise in terminal ballsitics, I have been reading and researching the topic for decades, and have read the primary lit via FBI, Feckler etc. so those links didn't teach anything I'm not awares of. As far as what you posted, one link I had already read, stated dependent bullet design, the softer expanding bullets require lower velocities for that tissue damage which are (wait for it...) well within the velocity of the 7.5-12" when using such a bullet.

I do think the only legit topic at hand here in terms of say 7.5 vs say 10 for my already outlined needs could be that temp vs perm wound stretch cavity, and I have not seen terminal ballistics data on the specific ammos, barrel length, distances, etc in this discussion.

A huge benefit to a rifle is the temporary wound cavity generated by cavitation with a cartridge traveling over ~2400 FPS.

A whole bunch of data points here:

https://www.rathcoombe.net/sci-tech/...s/methods.html



And some here:
https://www.bevfitchett.us/gunshot-w...listics-1.html

[WillBrink]

Well, according to your second link:
It is the author's belief that rather than there being a critical velocity above which the severity of wounds increases dramatically, there is instead a critical level (amount) of kinetic energy loss above which tissue destruction becomes radically more severe. This level is different for each organ or tissue. When a bullet or missile exceeds this kinetic energy threshold, it produces a temporary cavity that the organ or tissue can no longer contain, i.e., one that exceeds the elastic limit of the organ. When the elastic limit is exceeded, the organ "bursts." For full metal-jacketed rifle bullets and steel balls to reach this critical level of kinetic energy loss, these missiles must be traveling at very high velocities (greater than 800 to 900 m/sec; 2625 to 2950 ft/s). For soft-point and hollow-point rifle bullets, however, the same loss of kinetic energy will occur at lower velocities as a result of the deformation and breakup of the bullets. Thus, in the author's experience, for hunting bullets the critical velocity, appears to be between 1500 and 2000 ft/sec (457 to 610 m/sec).

Yup, that's what I was alluding to in my response, but I'd already read that source and many others over the years to come to such conclusions.

[Eurodriver]

I've read your reasons for why you want 5.56, and for why you want a short barrel. What you haven't explained is why you want such a short barrel that you are unanimously being told you're sacrificing too much ballistically for a lot of extra noise and concussion - so much so that many suppressors will literally be destroyed with too much firing out of a 7.5" which is the entire premise of this thread.

You tell me to screw off because I had to assume you wanted 7.5 because it looks cool - well, you didn't really give us a reason for the 7.5" length in particular. When asked, you sent a youtube video from someone we've never heard of. Are you bringing this in a vehicle with you? Pinellas County SWAT uses 7.5" barrels and they are trying to ditch them as fast as possible. Their all concrete range is destroying eardrums (figuratively, but probably literally as well) for no benefit.

[WillBrink]

I was going to say the same thing, but you said it better. Velocity is what makes rifles so much more effective than pistols. If you're giving up all that velocity, may as well go with a PCC, IMO.

That depends to a great deal on how much velocity you are giving up, what bullet you're using, distances, and other variables. Read the articles he supplied, and the comment from one author in post #25 from the author...