The new gearbox weapons will not be mentioned as they always spawn with the same parts. So long as it is the level you need it to be, it will be perfect.
I have not personally made any of the given images. All have been provided from the gearbox forums, the official borderlands wiki, or the online bl1 modding resource. Links to these great resources will be provided in the section at the very bottom.
While this guide cannot hope to condense and capture every bit of information found in these great community resources, this is meant to be a medium-depth reference point for those on steam, and quickly available for those trying to find information through the steam overlay WHILE playing the game.
I'm making this guide so I can explore the pros and cons of each part, and document my conclusions about their individual practicality. Included in the links located at the very bottom of this guide is a ported gearbox forums post which includes details about each part, screenshots, and zoomed images of each part from multiple angles, as well as a lot of the things I cover here in this guide.

That forum post was instrumental in helping me understand each part and identify my guns. If you find yourself looking for something I have not yet added to the guide, that post is a great place to start. I may eventually include a guide here on helping identify parts, but my hope is that as I add more and more content, I also find ways of making what I already have here accessible and concise.

Items have an overall "paint job" that determines manufacturer multipliers. The better the material, the better the gun. In the image, materials start at the worst on the left, and best on the right. Some unique items have a unique material, like the spy. These unique materials offer unique attributes, and evaluating any of these items should be done based on the merits of the weapon itself, not compared to other materials found on non-unique guns.

As the guide continues, there will be some more infographics showcasing parts. Even though they include grips, it's important to note that these will not affect the stats of your gun. Every tediore smg will have the same grip. Every vladof launcher will have the same grip. ETC ETC, don't get hung up trying to find some different grip, it won't matter.

Unlike in BL2, parts are not particularly associated with brand identity(save for the grip, which exists EXCLUSIVELY to determine brand identity and offers no stat bonus of any kind) The overall trend for weapon parts is that most of the time, part 1 will be the weakest with a statistic penalty, and part 5 will be the strongest with large benefits with little-to-no penalty. In most cases its safe to assume that part5 will be the best, but I intend on going into detail about fringe cases on when it might be in the best interest for the player to use a weapon that has a different part other than part5.

Tech is a hidden statistic of elemental guns which limits whether or not the weapon will create a damage over time effect. To learn more, visit the wiki article about tech pools.[borderlands.fandom.com] Essentially, the more tech your gun has, the more instances of damage over time it can put out in a row.

There are 3 categories of SMGs to look out for:

1. Stingers (Single shot and double)
2. Double Anarchies
3. Elemental (Including Explosive)

Depending on which type you want, some parts may be better than others. NOTE: Some legendaries get their unique name/properties from some parts, which means those parts may or may not be mutually exclusive with what is the "best" part.

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body5 is pretty much the best in slot for smgs: "increases damage, greatly increases fire rate, reduces recoil, and adds a little tech" For non-elemental non-uniques, body5 is objectively the best. For elemental guns, body3 adds bonus tech at the cost of damage, but still maintains decent fire-rate and recoil like body5.
Almost every barrel has its own subjective use on an SMG. To decide which barrel is best, you should note how the game decides what is and isn't an "anarchy".

• An anarchy is a double accessory smg with a sufficiently low accuracy. What this means is
  hyperion guns can never be an anarchy due to their accuracy bonus, and the "better" barrels that boost accuracy can not be found on an anarchy. Barrel2 is the best for this as it has the lowest accuracy for a damage boost, but IF for some reason you would like to maximize the accuracy of your anarchy, riding the line of the highest possible accuracy, barrel1 has more accuracy than barrel2, but does not grant any bonus accuracy that would put it above the threshold. This "accurate" anarchy barrel will cost damage though.
  
• A stinger is more of an ammo efficient way of using your smg ammo, but still there's some wiggle room here for what you want. For maximum base damage (without giving up fire rate), barrel4 will give you bonus damage at the cost of accuracy. For maximum accuracy, barrel5 is the best choice.
  
• Elemental weapons gain bonus utility from having extra tech. Hopefully at some point I can do enough testing to figure out the best combination of parts for an elemental weapon. In theory, the highest tech amounts make a pretty big trade-off for other stats, but idk what the best trade-off is. Barrel3 has a tech boost with no penalties other than just not being as good as the others. (Barrel3 on a non-elemental gun gives weird spiraly bouncy bullets. These are awful, avoid them)

The main trade-offs each magazine offers are reload speed vs capacity vs tech bonus. If you're not using an elemental smg, you probably don't care about the tech bonus. Mag5 is the best compromise of all of these. The mag4 drum mag offers the highest capacity for no tech and the slowest reload speed, good for anarchies to achieve higher sustained DPS. Mag3 gives bonus tech over Mag5, has additional capacity (not as large as mag5 or mag4), and has the fastest reload speed, so you can't go wrong with mag3, but it especially shines (pun intended) on elemental weapons.
Accessories are pretty self-explanatory for smgs. If you want an elemental smg, you need the elemental accessory for that element. If you want a double stinger, or double anarchy, you need the double accessory. The only point at which you should be deliberating on an accessory is for a non-elemental stinger, and at which point, there is no better accessory than acc2. NOTE: There is more than one acc2, "ruthless" and "vector". Vector is what you're looking for, and can be noted by a prefix on your weapon of "Malevolent".
No stock will give you the highest reload speed, if that's what you care about, but stock5 "gives the largest recoil reduction and adds tech." with neither boost nor reduction in reload speed. The trade-off is reload speed for recoil reduction, although stock5 is still not cumbersome in any way.
For SMGs, which sight you have on your gun will not affect the stats on your gun in any way. Do not avoid picking up or using a particular SMG just because you don't like the sight. Which sight is best for you will be entirely subjective.
When a gun has statistics (based on its parts, material, and brand) within certain thresholds, it may be granted a special title. When such guns are granted a title, they are given bonus statistic boosts. As mentioned before, the only notable SMG subtypes are stingers and anarchies. As all uniques will overwrite titles, it's impossible to get a unique anarchy or stinger. The dreaded "thumper" comes from mag1, so you will never find a thumper wildcat, spy, or bone-shredder.

• The hellfire gets its unique property from its accessory. All other parts can vary, but as such as it is locked in its acc, you will never find a double/anarchy/malevolent hellfire. It should be noted that the hellfire's unique properties give such a boost to tech, further tech boosting parts may be redundant.
  
• The b***h gets its unique property from its barrel, but has an unlisted EXTREME boost to tech. (Not as high as the hellfire or tsunami, however) Elemental variations of this smg will over-perform compared to non-unique variations.
  
• The savior gets its unique property from its body, which means it might have less damage and fire-rate than a non-unique with body5, but the savior version of body3 has an unlisted boost to tech, even greater than body3. While not as great as a b***h for elemental weapons, elemental saviors will prove to be exceptional. NOTE: in case you didn't know, it is possible to get a savior body on a bone shredder. You should look for a glowing body like body3, and ammo regeneration should be listed on the item card. A bone shredder is a very poor choice in smg however due to its locked accessory and barrel.
  
• The wildcat gets its unique property from its magazine, and has the downside of -200% accuracy. However, this magazine is a variation of a drum mag with great tech boost. An elemental wildcat has potential to be exceptional.
  
• The Tsunami gets its unique property from its accessory, with all other parts being variable. It should be noted that the tsunami's unique properties give such a boost to tech, further tech boosting parts may be redundant.
  
• The spy gets its unique properties from its material and sight, but almost all other parts are locked. The only variable parts are the body and accessory.
  
• The typhoon gets its unique properties from its accessory. Other than its unique material, all other parts can vary,

While the game does contain 5 options for body and mag types, unlike other weapon types, not all sniper rifles can contain every variation of that part. The game separates snipers into two distinct categories:

1. Faster firing "Semi-Auto"

• Limited to body2 OR body5
  
• Limited to mag2 OR mag5
  
• Limited to Dahl, S&S, Torgue, or Hyperion

2. Harder hitting, slow "Bolt-Action"

• Limited to body1, body3, OR body4
  
• Limited to mag1, mag3, OR mag4
  
• Limited to Atlas, Jakobs, Maliwan, or Vladof

Depending on which category of rifle you have, effects of accessories might vary.

Regardless of which category the rifle is in, it still:
1. Will have the same "manufacturer grip" (which again, affects the gun's stats in NO way).
2. Can have any one of the 5 barrel types (shared between both types).
3. Can have any one of the 5 stock types (shared between both types).
4. Can have any one of the 5 sight types (shared between both types).

NOTE: Some legendaries get their unique name/properties from some parts, which means those parts may or may not be mutually exclusive with what is the "best" part.

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Body5 is the best body a "semi-auto" rifle can spawn with, with objectively superior stats over the only other option for "semi-auto" rifles to have.
Which body is best for a "bolt-action" depends on what the goal is. Body3 adds a lot of tech, and has stat bonuses which try and mimic semi auto rifles: "slightly decreases damage, increases fire rate, reduces recoil, and adds alot of tech". Body4 doubles down on being a bolt action and "increases damage, reduces fire rate, adds alot of recoil, and adds a litte tech."
Which barrel is best depends on if you're looking for more accuracy, or more damage. Barrel4 offers the highest damage bonus(+30%) as well as a bonus to tech, compared to barrel5 which offers half of the damage bonus(+15%) but an EXTREME 130% bonus to accuracy. While barrel2 has a significant accuracy penalty with the same exact damage bonus of barrel5(which is the MOST accurate barrel), there is a slight use for this part. Vladof gamble rifles
get a boost to both damage and tech, BUT because the qualification for a "gamble" title, the rifle must reach a threshold of a poor enough accuracy.
Mag5 is the best mag a "semi-auto" rifle can spawn with, with objectively superior stats over the only other option for "semi-auto" rifles to have.
Which mag is best for a "bolt-action" depends on what the goal is. Mag3 adds a lot of tech and increases damage, but reduces mag size significantly. Mag4 increases fire rate and reload speed, but offers no bonus to damage or tech.
What accessory is best for a sniper rifle depends on what the goal is. If you're looking for an elemental rifle, this will be mutually exclusive with any other bonus an accessory can give you because the element comes FROM the accessory.
For a non-elemental rifle, acc4 is the best granting an 30% damage boost.
For a non-elemental rifle, acc4 is the best granting an 18% damage boost.
Which stock is best depends on if you'd like to squeeze in just a tiny bit of extra tech into your elemental rifle. Stock3 has the second lowest recoil reduction, but gives +1 to tech. For non-elemental guns, stock5 offers a 100% bonus to recoil reduction and has no penalties.
While the sight is mostly subjective, depending on what zoom level you'd personally prefer, one sight DOES offer a slight statistic boost. Sight3 offers a slight tech boost. For non-elemental rifles, this will not matter.
When a gun has statistics (based on its parts, material, and brand) within certain thresholds, it may be granted a special title. When such guns are granted a title, they are given bonus statistic boosts. All unique items overwrite the title of the gun, so it is impossible to get a unique/legendary with one of the following boosts:

• Thunder - Granted to sniper rifles with high base damage, gives no extra bonus
  
• Gamble - Granted to sniper rifles with low accuracy (< 94.7), giving the weapon bonus tech and critical hit damage. (Can only be found on jakobs and vladof rifles)
  
• Hawkeye - Granted only to bolt-action sniper rifles with high accuracy (>= 98.7), giving the weapon reduced spread and slightly higher zoom. (Vladof's passive material bonus increases spread beyond this potential threshold, so there will never be a vladof hawkeye)
  
• Wrath - Granted only to semi-automatic sniper rifles with a high enough quality and a large enough tech pool, giving the weapon bonus damage and even further bonus tech.
  
• Lance - Granted only to semi-automatic sniper rifles with a high enough accuracy, giving it an increase to damage, knockback and tech. (In theory, this could apply to any sniper rifle within the threshold, but realistically, this has been found to only be applied to hyperion rifles with barrel5)

• The volcano gets its unique property from its accessory, with an unlisted MASSIVE boost to tech(+16). Further boosts to tech may be redundant.
  
• Surkov has a locked barrel.
  
• Skullmasher has a locked barrel.
  
• Cyclops has a locked sight, and offers a hidden large boost to tech(+3). Elemental cyclops rifles will most likely over-perform compared to a similarly parted non-unique. An elemental cyclops might still need compromises on other parts for tech boosts to be a competent elemental weapon. NOTE: in case you didn't know, it is possible to get a cyclops scope on a kyros' power. Look for the massive sight5, and a listed 3.2x weapon zoom. The kyros's power has locked accessory, but does not have any other part restrictions.
  
• The cobra gets its unique property from its accessory, with an unlisted HUGE boost to tech(+12). Further boosts to tech may be redundant.
  
• The orion gets its unique property from its accessory, with an unlisted LARGE boost to tech(+10). Further boosts to tech may not be needed, but it does not get the same level of boost to tech as the volcano, or cobra.
  
• The penetrator has a locked barrel that has a small boost to tech (+2), which is comparable to barrel4, however barrel4 has an additional 30% boost to damage that the penetrator barrel lacks. NOTE: in case you didn't know, it is possible to get a penetrator barrel on a reaver's edge. Look for the shape of barrel1, but with a large fire rate AND a large magazine.
  
• The executioner gets its unique properties from its mag, with an unlisted boost to tech(+1), meaning an elemental executioner has potential to be exceptional.
  
• The bessie gets its unique attributes from its sight, but being a jakobs, cannot be elemental. All other parts on the bessie are variable, but is unable to have the skullmasher barrel.
  
• The nailer gets its unique properites from its barrel, but having a locked barrel, sight, body, and mag, only the accessory and stock are variable. Unfortunately, given the locked parts, even with the stat bonus of the unique properties, it is unlikely a nailer will prove more effective than a non-unique sniper with better parts.
  
• Whitting's Elephant Gun gets its unique properties from a unique sight that is a non-scope. It has a locked barrel, sight, mag, and stock. The body and accessory are variable, but given it is a jakobs, it is impossible to get an elemental variant.

I could have not found or made any of this without the help of the community, here are some super helpful links.

In depth stats and descriptions on every single part [blmodding.wikidot.com]

Determine whether or not your gun has the parts you're looking for[forums.gearboxsoftware.com]

Mix and match parts for a theoretical gun and see what it would look like[gearcalc.50webs.com]

The best deep dive on elemental damage I've seen (link uses waybackmachine)[web.archive.org]

Wiki page about pearlescents[borderlands.fandom.com]

Wiki page about unique items[borderlands.fandom.com]

Wiki page about legendaries[borderlands.fandom.com]

Hub page to wiki articles about all weapon types[borderlands.fandom.com]

Lootlemon-some new website I found, haven't dug into it yet[www.lootlemon.com]