I have been asked a # of times about barrel length and accuracy.
So a week last Sunday (actually a while ago now) I did an informal study.
Using Chaszel 357 mag inserts of 6", 11", and 18" and S&B 158 Fmj cartridges.

I tried the 18" at 50 yds, (it was not the same one I did the previous review with).  The results were at 3.015"  for 3 shots, so I switched to 25 yds.  I should mention that the 18" 357 magnum insert had been rechambered to 357 maximum (yes I know not an entirely fair comparison).   Rechambering is not condoned by Chaszel and may have had an effect on accuracy because of the bullet jump, .....or not. (update; Chaszel is now offering a 357 max insert and we will be bringing some in)
Here are the results of a single 5 shot group with the S&B cartridges mentioned

18" barrel 1.890"
11" barrel 2.550"
6" barrel 2.040"

So with one go at it,  I would say the results are inconclusive.  Or we can jump to the conclusion that barrel length makes no difference.....

The 6" had a much larger muzzle flash indicating, as expected, that some of the powder was unburnt.  I expect the longer barrels have more speed and therefore more energy, other than that there was not much difference with the cartridge/inserts/chambering tried.  

Different cartridges or handloading may change everything.  So, more work is required, oh darn, I guess I will have do some more shooting.... 

Bottom line, buy what you need/want.  If you need portability go 6" if you need more energy, go 18".  If you are going hunting for a deer size animal a 3" group at 50 yds is probably adequate.   Not sure there is always the compromise 11" fairly easy to stow and more power than 6".

This article is provided for information only your results may vary.  No liability is assumed in providing this information. 
 This article is copyright protected and may not be used without permission.

So the first batch of Chaszel inserts arrived.  I like to be able to speak knowledgeably to customers about the products I sell so of course I had to try one out.
First of all it is important to keep in mind that if you choose to use one of these inserts you should do it in a shotgun capable of handling the force generated by the cartridge you choose to fire.  Chaszel’s own literature and website caution that your first shot should be done remotely just in case the firearm decides to explode.
I chose a 12 gauge 18” 357 mag. insert to try out.  The shotgun I chose was a H&R SB1. Tracker rifled barrel shotgun.  I chose it for a couple reasons; this frame was used by H&R for the basis of their 357 and 44 mag rifle so I was confident that it was suitable for the pressure and second it already had a scope mounted so aiming should be a little more consistent.

The Chaszel insert has 3 different o-ring grooves of different diameters at the muzzle end so that you can change the position of the o-ring to get a tight fit in the bore of your shotgun.
There is also an o-ring at the chamber area.  The o-rings center the insert in the bore but also act as a buffer to prevent scratching of the shotgun bore.

The 18” insert is not a lightweight weighing just over 1  ¼ lbs.
The insert has 2 scallops on the side of the chamber to allow the user to fish out the fired shell casing.  Extraction was easy with the limited rounds fired.
How does it shoot? 
I placed a large pc. of cardboard and 3 aiming stickers out at 50 yds.  I am glad I used the large pc of cardboard because my first shot was about 16” high and almost off the page so to speak.

I moved the aiming stickers down toward the bottom of the cardboard.  I made no scope adjustments as I was only interested in the relative accuracy of the insert.  I had 3 different brands and 2 different weights of bullet in 357 mag.  They were Winchester 110 gr, CCI 158 and S&B 158 gr.  I was only interested in doing a quick check of what a barrel insert might perform for me.
Shooting was done from the prone position.  Temp was about 0 deg C, there was no real wind to speak of.  The scope was set to 4.5 x and probably should have been cleaned.  Aiming was done pointing at the center of the white blob at the bottom of the cardboard as the winter twilight was starting to set in.  Shots were fired one after the other, 3 shots at one target with one type of ammo then 3 at the next and so on.   All the groups landed about 12" to 14" high with the Winchester ammo centered the CCI slightly off center and the S&B off to the right about 5 1/2" but close to the same height.
Whether the drift to the right was a function of the insert heating up or the change in ammunition is unknown.  Seeing that the S&B group was the smallest of the 3 shot groups, I am inclined to believe it is a function of the different ammo.

Conclusion
I didn’t know what to expect but aside from being way off the aiming point (which is why there are sights and scopes with adjustments) I was pleasantly surprised with a 3 shot grouping between 1 ½” to 2”.  At 50 yds. The impact area between the 3 brands of ammo was relatively close vertically with the exception noted above.  Granted I chose the 18” version which may be inherently more accurate but I have seen plenty of handguns with their short barrels turn in some exceptional accuracy.  I have to admit that I want to try rotating the liner in the barrel to see if that alters the point of impact.  Although your setup may yield different results, if you want some extra fun with your shotgun this is one way to do it.  If you have one of the typically inexpensive single shot shotguns hanging around and you want a pistol caliber carbine for plinking or small game hunting this is a relatively inexpensive way to make a dedicated or convertible unit.
Update Sept. 2, 2024
I recently took out a 30-30 insert and was using up some left over ammo and was able to get what I thought, were impressive groups, considering it is a barrel inside a barrel suspended by O-rings. 
With one brand of ammo (Win. 150 gr) at 25 yds a 1/2" group (3 shot) and a different brand ammo (Fed. 170 gr) 1.190" at 50 yds (3 shot)  This is way better than some of my previously reported pistol caliber groups.  Perhaps the rifle calibers have better potential accuracy?  Since that Ammo is now gone I do not know what kind of performance I will get from a different brand or if it is repeatable.  When I get a chance I will have to find out.

This article is provided for information only, your results may vary.  No liability is assumed in providing this information. 
 This article is copyright protected and may not be used without permission.

A customer once asked
While I have your attention, perhaps I can ask for your opinion / expertise. My plan is to use the insert in a Mossberg HS 12. This firearm is Turkish made. I’m aware there’s about 12k psi difference between .44 mag and .45-70. Do you think it is smarter / safer to go .45-70 instead? And do think Turkish made shotguns are capable of handling the insert?
My response
Hi Customer
Wow, that is a way more complex question than it seems on the surface. 
I started to respond and realized that I had answered similar questions a # of times with people asking about different firearms.  I think that I will consider you the first reader of this response and will publish it on the website for others to chew on. 
Chaszel makes very specific recommendations on their website in regard to their recommended choice of firearms to use their inserts in.  That information is the only reliable advice that NINE35 can endorse.  It is strongly encouraged that the suggestions of Chaszel, especially in relation to remote firing to test a firearm are followed.  Since NINE35 has no control over the condition of the firearm or ammunition used, no liability is accepted in selection of appropriate firearms for use with shotgun caliber conversion inserts.
 The following information is presented in order to provide anecdotal information in order to help understand some of the considerations of why I think Chaszel made their recommendations.  
Factors to consider.
1st/ What is the condition of the firearm?  Is it new and tight, or worn out, have any parts started to fatigue?  What is the headspace condition?  While this is not mentioned by Chaszel, the obvious course of action would be, do not use any firearm that is not in safe condition, even if it is on the recommended list.
2nd/ What is the design? For instance, a question regarding the quality of Turkish (insert any brand or country here) manufacture was raised.  In my opinion the design is as important.  Is a Turkish, fold in half, single shot, as strong a design as an over/under? I do not think so but it could be.  To do the assessment we need to look at things called load paths and sizes of components such as the hinge pin dia. frame dimensions, materials used etc. and how the force is applied.   For instance, in an O/U which may have lower lugs engaging the frame, the force between the breech face and the locking bolt/frame lugs are likely at a better mechanical advantage than the top barrel enjoys.  Many rifle shotgun combos have the rifle barrel located on the bottom, could it be for this reason?  This does not discount the fact that the upper barrel is still designed to handle the forces involved in firing a shot shell.  The upper barrel is in a similar position relative to the hinge pin and lower bolt as many single shots such as a Cooey or similar firearm.   
Although it is not possible to cover every design, one problematic issue has been clearly identified and that is the issue of the size of the firing pin hole relative to the primer.  With a rifle the pin and hole are relatively small, effectively supporting the primer as best as can be.  Typically, this is not so with shotguns, they operate at lower pressures where this is not as much of a consideration.  There is (or was) an internet video where the author describes how the primer ruptured and blew the firing pin out the back of the firearm (Turkish made folding shotgun).  It pushed past the hammer becoming lost forever.  Fortunately for the owner it was being remotely fired at the time.  For this reason, shotguns that have been factory designed to handle centerfire rifle and pistol cartridges, as well as shotshell cartridges, are clearly a better choice.  Examples are H&R/NEF, SB1 (lower power calibers) and SB2 frames, TC Encores and some of the later Rossi's that came as multi barrel firearms (the models which included high power rifle calibers) from the factory,  I also like the Baikal firearms as the seem to share the same frame as the single shot rifle.
3rd/ What is the period of manufacture and quality?  For instance, the Iver Johnson, Cooey, Stevens 301 and many similar designs, use a 3-sided frame a full width hinge pin with a swinging bottom lug/bolt engagement.  I would put my money on a new Stevens made in China (not a fan of the country or politics, but know many good people from there) over an old cast iron Iver Johnson.  If we look through the lens of #2 (design) I may take a Cooey over a folding Turkish modern.  I have heard of one person who had an issue with a Cooey (it did not fail) but the shell peened itself into a soft breach face.  They had removed the spring-loaded ejector and believe that created a situation where the insert was not held against the breach face.  Not having personally inspected the firearm, I cannot say if there were other issues such as a loose hinge pin or headspace problems (their theory seems to support this).  Do not forget quality of manufacture.  For instance, the Cooey just mentioned, may have had the relatively thin factory case hardening worn through over time, remember the ejector rides over it when closing, or, maybe it was not properly heat treated in the first place.   Technology has vastly improved in both materials and processes so with similar designs, the more modern firearm is likely the better choice.  With the Cooey motioned above, there were no injuries as there were no catastrophic failures even after approx. a box or so was fired through it.  The user did discontinue using it.  I have a customer with the aforementioned Stevens shotgun who has shown me pictures of shell casings showing obvious displacement of the primer into the area of the firing pin hole.   This leads us into #4 
4th/ Operating pressure and choice of cartridge / ammunition.  This is an important consideration as base diameter and operating pressure of the cartridge influence the real forces acting on the firearm.  The pressure and diameter, relate to what is commonly referred to as Bolt Thrust.  In single/double shotguns this is the force exerted between the hinge pin, locking bolts and breach face.   So, although the 44 Mag at 36 KPSI pressure sounds more powerful than a 12 ga at 12 KPSI, the force is less (using .43 dia. and 36KPSI for the 44 Mag. vs .73 dia. and 12KPSI for a 12 Ga. to calculate (pi x r2 x psi) than the 12 ga.  However, the force is more concentrated on a 44 Mag because of the much smaller dia. of the cartridge base.  Remember the peened Cooey breach face mentioned earlier, that is a good example of the wrong cartridge for that individual firearm, I believe the owner was shooting 357 Mag.  If 38 special loads had been used there may have been no issue as the force would be lower due to the lower operating pressure acting on the same diameter base.  This is a good time to remind people that firearm chambers should be dry (no oil) when firing.  The variation in potential ammunition used, also needs to be taken into account. For instance, 45-70 loads can be had in several different pressure levels, make sure you are using the trap door rifle load level cartridge pressures when using an insert, not those loaded to higher pressure for the modern 1885 lever firearms.  Components used in the manufacture of the cartridge have an effect.  As mentioned, we know that the primer/firing pin hole relation is one of the critical areas.  Choosing a cartridge with a harder primer will be better able to deal with the unsupported area.  Hand loaded cartridges are another similar topic as well.  There are so many variables that it is totally, a use at your own risk proposition.   However, if you are going to reload, I would suggest using rife primers in your handgun cartridges as well.
So, in summary
- Only Chaszel recommendations can be endorsed
Things to consider in purchasing a shotgun insert
-Is the shotgun a robust design?
-Overall condition and age of the firearm.  A 22LR may be safe in almost any shotgun, but if there is an excessive gap between breech face and insert and the shell rim ruptures upon firing, it may not affect the firearm but the resulting shrapnel may cause injury or worse.
-Era of manufacture and the potential for poor quality materials being used, it is my opinion that older Chinese firearms (almost any older Chinese manufactured product) was inferior compared to newer Chinese manufacture.  This can even apply to NA products, remember the term "cheap knockoff", looking the same does not = being the same.  For instance, the newer Chinese Stevens look like an SB1 but the larger firing pin hole on the Stevens indicates that it is not exactly the same even if it is robust in manufacture. 
-Design of the firearm related to the power level of the intended cartridge.  Think about if a primer ruptures with 36 KPSI pushing on it, where will the shrapnel or displaced firing pin and escaping gas go?
Specific answer to your question.
Many major brands such as Mossberg use Turkish manufacturers and would insist on the firearm meeting their standards.  So I believe that there are some good Turkish units out there.  As per Chaszel's remarks if it is other than the recommended shotguns listed, caution should be exercised.
Regarding your suggestion of choice between the 44 mag and 45-70 see the comments and calculations above.  The 45-70 has a larger base and bore dia. than the 44 mag and you may find the force exerted is not that great a difference as you may think.  The 45-70 benefits from having a rifle primer as standard, compared to a magnum primer (typically harder than standard pistol).  Will it be safe for the operator, probably, remember the remote firing caution from Chaszel.  Will it damage the firearm like the cautionary tales mentioned above?  It will depend on all the factors mentioned above.
I know that this is not the absolute yes or no you are looking for all I can offer is the following is what I have and use.
In my Turkish o/u and Rossi 20 ga. single, 9mm no issue so far.
In my NEF SB1 single, 357 mag and 357 Max. (custom reamed) , no issue so far.
Ron
This article is provided for information and only you can determine the factors discussed and how they apply to your firearm, no liability is assumed in providing this information.  If you are not sure bring it to a qualified gunsmith.
 This article is copyright protected and may not be used without permission.